avcodec/flicvideo: Check remaining bytes in FLI*COPY

Fixes: Timeout
Fixes: 37795/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLIC_fuzzer-4846536543043584

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5f835efb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>