Since commit 676a395a aac->frame->data is not necessarily allocated at
the end of aac_decode_frame_int if avctx->channels is 0.

In this case a bogus frame without any data, but non-zero nb_samples is
returned.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>