ad_ffmpeg: fix crash by wrongly sync'ed AAC

SEGV due to insufficient allocation of AVFrame data in libavcodec/utils.c::ff_get_buffer().
When ffmpeg mis-detected AAC frames in aac_decode_init(), it set wrong sample format.
(default sample format had been changed to float, planar).
This caused the bug in avcodec_default_get_buffer2()->update_frame_pool().