qtdemux: Increment current stts index in all code paths after reading one chunk

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 +8545,7 @@ done2:
           /* save values */
           stream->stts_time = stts_time;
           stream->stts_sample_index = j + 1;
+          stream->stts_index++;
           goto done3;
         }
       }