avformat/mov: fix avg_frame_rate calculation

Fixes integer overflow
Fixes Ticket3390
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -3419,7 +3419,7 @@ static int mov_read_header(AVFormatContext *s)
         }
         if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO && sc->nb_frames_for_fps > 0 && sc->duration_for_fps > 0)
             av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
-                      sc->time_scale*sc->nb_frames_for_fps, sc->duration_for_fps, INT_MAX);
+                      sc->time_scale*(int64_t)sc->nb_frames_for_fps, sc->duration_for_fps, INT_MAX);
     }
 
     if (mov->trex_data) {