The invention relates to an airbag device for a vehicle occupant restraint system comprising an inflator, a folded airbag located within a housing, a release means for releasing the airbag and an ignition means for igniting the inflator. An airbag device for a vehicle occupant restraint system is known from WO 97/24405, in which the airbag is ignited by means of a pyrotechnic propellant that is located between the folded airbag and the inflator and that ignites when the pressure is lowered after triggering the release means. The ignited propellant generates the required pressure for deploying the airbag. Such an ignition by a propellant has the disadvantage, however, that it is not very well suited for safety-oriented deployment because an inflator igniting by means of a propellant is designed to inflate the airbag rapidly but, in the event of misfiring, not to inflate it at all. This can result in an insufficient restraint of the vehicle occupant. A pyrotechnic is also known, from EP-B1 0 566 771, which is ignited by means of a gas generator. This pyrotechnic does not, however, serve as a propellant for inflating the airbag but instead serves as a pyrotechnic inflator for igniting the gas generator which is, in turn, ignited by a gas generator. The object of the present invention is to provide an airbag device having an ignition means for inflating a folded airbag in which the airbag is inflated rapidly but securely and without an unnecessary gas flow from the gas generator. This object is solved by an airbag device of the aforementioned kind that has a safety valve that releases the gas from the gas generator to the surroundings of the airbag before the ignition means ignites the inflator. This results in that, after ignition of the ignition means, the pressure in the gas generator is reduced by releasing the gas in the gas generator via the safety valve into the surroundings of the gas generator, i.e., out of the airbag, before the gas flow generated by the gas generator during its ignition contributes to the inflation of the airbag. In particular, a pressure reduction can be achieved in a simple way in that, before igniting the gas generator, the safety valve is opened in the direction of the interior of the airbag, i.e., from the surroundings of the gas generator towards the interior of the airbag, and/or in that the supply duct leading from the gas generator is opened to the surroundings of the gas generator, so that the gas is released from the gas generator into the surroundings. The effect can thereby be achieved that, during igniting of the gas generator, the pressure in the gas generator decreases gradually before this gas is used for inflating the airbag and the inflator. As a result, the rate of gas flow required for inflating the airbag can be attained in a simple way without any problem in the case of an ignition misfire. The pressure reduction achieved in this way ensures that the inflator ignites and inflates the airbag in a controlled manner with a reproducible and predictable timing because the inflator, which has already been ignited, must be ignited only after a defined minimum gas pressure is reached, which can be calculated. By separating the gas generation from the ignition, a rapid inflating of the airbag, for example, in a case of a fire is made possible since the ignition of the gas generator can be delayed in relation to the inflating of the airbag. The delayed ignition of the gas generator can be achieved, for example, by using a smaller propellant in the gas generator. In particular, it is possible in this way to prevent ignition of the gas generator by the flames of the fire before or together with ignition of the gas generator. The above object is further solved by an airbag device of the aforementioned kind that has a safety valve that releases the gas from the gas generator to the surroundings of the airbag before ignition of the gas generator. In this way, it is achieved that, after ignition of the gas generator, the pressure in the gas generator is reduced by releasing the gas from the gas generator into the surroundings via the safety valve before the gas flow generated by the gas generator during its ignition contributes to the inflation of the airbag. As described above, in this way, it is achieved that the pressure in the gas generator is reduced before the gas generation in this gas generator starts. In particular, it is achieved that, as a result of the pressure reduction, the risk is avoided that the gas generator is ignited before the gas generator generates a sufficient amount of gas in order to inflate the airbag, so that the airbag will not inflate. The safety valve may release the gas into the surroundings of the gas generator, into the airbag, into the vehicle, i.e., into any surroundings which, in a case of an accident, can be filled with gases. The safety valve is preferably constructed in a simple way in that a gas inlet port is formed in the housing and the safety valve is arranged downstream from the gas inlet port and in that, when the gas is released, it is provided that a part of the folded airbag enters into the gas inlet port. When, during the inflation, a pressure is applied to the folded airbag, the folded airbag is pressed against the gas inlet port, in which case the gas inlet port opens. Thus, a release of the gas is achieved, in which case a part of the folded airbag reaches into the gas inlet port and prevents a total filling of the airbag with the inflating gas. In order to ensure that the safety valve, after it has released the gas, closes again as quickly as possible, it is preferred that the safety valve has at least one movable valve body, which is preferably constructed as a flap. It is thereby achieved that, in the case of a gas release, the gas can escape in a simple way through the safety valve into the surroundings. A secure closing of the safety valve is achieved in that a guide and/or a flap body is provided as an integral component, which closes the safety valve when the gas pressure is low, preferably below 10 mbar. The safety valve should be suitable for a rapid closure, for example, in a case of an accident. This can be achieved by providing the safety valve with at least one flap body that is moved relative to the housing and/or that is formed so as to be elastically deformable and by providing the safety valve with a gas-conducting structure that prevents the flap body from unintentionally closing the safety valve. In addition to a flap-type safety valve, it is preferred that the safety valve