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>
(cherry picked from commit 9a0d36e562)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpsubdec.c

@@ -106,6 +106,9 @@ static int mpsub_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &mpsub->q);
 
 end:
+    if (res < 0)
+        ff_subtitles_queue_clean(&mpsub->q);
+
     av_bprint_finalize(&buf, NULL);
     return res;
 }