qtdemux: Increment current stts index whenever we finished one stts entry

Otherwise we could read more chunks than there are available, doing an
out of bounds read and potentially crash.

https://bugzilla.gnome.org/show_bug.cgi?id=777469

gst/isomp4/qtdemux.c

@@ -8545,6 +8545,8 @@ done2:
           /* save values */
           stream->stts_time = stts_time;
           stream->stts_sample_index = j + 1;
+          if (stream->stts_sample_index >= stream->stts_samples)
+            stream->stts_index++;
           goto done3;
         }
       }