avformat/mov: Don't add attached pic if one is already present

Fixes: memleak
Fixes: 67714/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-5671570999476224

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

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 9d219ff149)

libavformat/mov.c

@@ -8820,7 +8820,7 @@ static void mov_read_chapters(AVFormatContext *s)
 
         if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
             st->disposition |= AV_DISPOSITION_ATTACHED_PIC | AV_DISPOSITION_TIMED_THUMBNAILS;
-            if (sti->nb_index_entries) {
+            if (!st->attached_pic.data && sti->nb_index_entries) {
                 // Retrieve the first frame, if possible
                 AVIndexEntry *sample = &sti->index_entries[0];
                 if (avio_seek(sc->pb, sample->pos, SEEK_SET) != sample->pos) {