avcodec/pafvideo: Check packet size and frame code before ff_reget_buffer()

Fixes 1745/clusterfuzz-testcase-minimized-6160693365571584
Fixes: Timeout

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