If the first guessed pts is less than the start_pts, it could
lead to a negative PTS being returned.
Change the behavior so that the first frame's pts, if zero, is
set to the start_pts. If more than one frame is less than the
start_pts, the start_pts is determined invalid and set to 0.
Valid start_pts example:
start_pts = 500
first frame (pts = 0)
pts = 500 (< start_pts)
pts -= 500 (offset by start_pts)
ret 0
second frame (pts = 700)
pts = 700 (no change, > start_pts)
pts -= 500 (offset by start_pts)
ret 200
Invalid start_pts example:
start_pts = 500
first frame (pts = 0)
pts = 500 (< start_pts)
pts -= 500 (offset by start_pts)
ret 0
second frame (pts = 300)
pts = 300 (< start_pts, start_pts set to 0)
pts -= 0 (start_pts is now 0)
ret 300
ff0c58e963