aac: Relax reserved_bit validation

Although the specification mandates this bit to zero, it may happen
that software tools incorrectly flip it to one, invalidating a possibly
valid stream.

Relax this restriction, by failing only when AV_EF_BITSTREAM is set.
This behaviour is similar to aac decoders in Firefox and Quicktime.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
