avformat/mpsubdec: Clear queue on error
Fixes: Memleaks
Fixes: 17219/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5720539124989952
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>