mov: immediately return from mov_fix_index without old index entries

If there are no index entries, e_old = st->index_entries is only one
byte large, since it was created by av_realloc called with size 0.

Thus accessing e_old[0].timestamp causes a heap buffer overflow.
Reviewed-by: Sasi Inguva <isasi@google.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/mov.c

@@ -2961,7 +2961,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
     int first_non_zero_audio_edit = -1;
     int packet_skip_samples = 0;
 
-    if (!msc->elst_data || msc->elst_count <= 0) {
+    if (!msc->elst_data || msc->elst_count <= 0 || nb_old <= 0) {
         return;
     }
     // Clean AVStream from traces of old index