In mov demuxer, check that stts data exists, fix crash, issue #2479

Originally committed as revision 26227 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mov.c

@@ -1515,7 +1515,8 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
     if (sc->time_offset) {
         int rescaled = sc->time_offset < 0 ? av_rescale(sc->time_offset, sc->time_scale, mov->time_scale) : sc->time_offset;
         current_dts = -rescaled;
-        if (sc->ctts_data && sc->ctts_data[0].duration / sc->stts_data[0].duration > 16) {
+        if (sc->ctts_data && sc->stts_data &&
+            sc->ctts_data[0].duration / sc->stts_data[0].duration > 16) {
             /* more than 16 frames delay, dts are likely wrong
                this happens with files created by iMovie */
             sc->wrong_dts = 1;