avformat/mov: Break out early if chunk_count is 0 in mov_build_index()

Without this some operations might overflow (undefined behavior)
even though the index adding loop would never execute

No testcase known
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -3900,6 +3900,9 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
     } else {
         unsigned chunk_samples, total = 0;
 
+        if (!sc->chunk_count)
+            return;
+
         // compute total chunk count
         for (i = 0; i < sc->stsc_count; i++) {
             unsigned count, chunk_count;