dashenc: check pts to prevent division by zero error

this usecase will cause a division by zero trap:

1. dashenc has received one frame
2. os->max_pts and os->start_pts have same value
3. delta between max_pts and start_pts is 0
4. av_rescale_q(0, x, y) returns 0
5. this value is used as denominator in division
6. Bang! -> segfault

this fix checks that max_pts > start_pts.
the fix has been tested and works.
Signed-off-by: Alfred E. Heggestad <alfred.heggestad@gmail.com>
Reviewed-by: Jeyapal, Karthick <kjeyapal@akamai.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/dashenc.c

@@ -1883,7 +1883,7 @@ static int dash_flush(AVFormatContext *s, int final, int stream)
                                                                   st->time_base,
                                                                   AV_TIME_BASE_Q));
 
-        if (!os->muxer_overhead)
+        if (!os->muxer_overhead && os->max_pts > os->start_pts)
             os->muxer_overhead = ((int64_t) (range_length - os->total_pkt_size) *
                                   8 * AV_TIME_BASE) /
                                  av_rescale_q(os->max_pts - os->start_pts,