avformat/mov: fix the check for the heif item parsing loop

Fixes: Null pointer dereference
Fixes: 67861/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5352628142800896

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 31327c2d07)

libavformat/mov.c

@@ -9427,7 +9427,8 @@ static int mov_parse_tiles(AVFormatContext *s)
                 break;
             }
 
-            if (k == grid->nb_tiles) {
+            if (k == mov->nb_heif_item) {
+                av_assert0(loop);
                 av_log(s, AV_LOG_WARNING, "HEIF item id %d referenced by grid id %d doesn't "
                                           "exist\n",
                        tile_id, grid->item->item_id);