avcodec/flashsv: add FF_CODEC_CAP_INIT_CLEANUP to flashsv1

Fixes: memleaks on error paths during init
Fixes: 15533/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLASHSV_fuzzer-5647977168764928

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