avcodec/ituh263dec: Check for the bitstream end in ff_h263_decode_mb()

Fixes invalid shift

Fixes: 670/clusterfuzz-testcase-4852021066727424

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