avcodec/mpeg4videodec: Clear bits_per_raw_sample if it has originated from a previous instance

      Fixes: assertion failure
Fixes: ffmpeg_crash_5.avi

Found-by: Thuan Pham <thuanpv@comp.nus.edu.sg>, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>