Fixes: memleaks on error paths during init
Fixes: 15548/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLASHSV2_fuzzer-6324019382452224

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