Fix unused return value

[Roms1383]

The crash happens when the function returns a value which is ignored on Redscript side.

For example, having defined this method:

public native class AudioSystemExt {
	public final native func RegisterEmitter(...) -> Bool;
}

Would work fine when used this way:

let registered = system.RegisterEmitter(...);

But would cause a crash when used this way:

system.RegisterEmitter(...);

[jac3km4]

lgtm!