deps-libff: Adjust start_pts if invalid pts found

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