avcodec/ac3_parser: Check init_get_bits8() for failure

Fixes: null pointer dereference
Fixes: ffmpeg_crash_6.avi

Found-by: Thuan Pham, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>