avcodec/flicvideo: Check frame_size before decrementing

Fixes: runtime error: signed integer overflow: -2147483627 - 22 cannot be represented in type 'int'
Fixes: 1637/clusterfuzz-testcase-minimized-5376582493405184

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