avcodec/pafvideo: Check for bitstream end in decode_0()

Fixes: Timeout
Fixes: 3529/clusterfuzz-testcase-5057068371279872

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