avcodec/vp56: Check for the bitstream end, pass error codes on

Fixes timeout
Fixes: 446/fuzz-3-ffmpeg_VIDEO_AV_CODEC_ID_VP6_fuzzer

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
