UBSAN: type conversion changes value in ivas_mdct_core_invQ()

In ivas_mdct_core_dec.c:529:

    noise_gen_mode_bfi = -1;

a negative value is assigned to a type which does only define positive values which changes the assigned value and is actually undefined behaviour. This does not have any impact, though, as noise_gan_mode is only accessed if it is set to an appropriate value. Still, to silency the sanitizer, introduce an undefined value for this enum type.