dashenc: check pts to prevent division by zero error

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>