avformat/mov: Skip stsd adjustment without chunks

Fixes: Assertion failure
Fixes: clusterfuzz-testcase-minimized-media_pipeline_integration_fuzzer-5683096400822272

Found-by: Clusterfuzz
Reported-by: Dan Sanders <sandersd@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 18a567c369)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -6002,6 +6002,7 @@ static int mov_seek_stream(AVFormatContext *s, AVStream *st, int64_t timestamp,
     }
 
     /* adjust stsd index */
+    if (sc->chunk_count) {
     time_sample = 0;
     for (i = 0; i < sc->stsc_count; i++) {
         int next = time_sample + mov_get_stsc_samples(sc, i);
@@ -6012,6 +6013,7 @@ static int mov_seek_stream(AVFormatContext *s, AVStream *st, int64_t timestamp,
         }
         time_sample = next;
     }
+    }
 
     ret = mov_seek_auxiliary_info(s, sc);
     if (ret < 0) {