Fix bug in --mv-constraint
Fix bug in --mv-constraint

Subpixel motion estimation return 0-vector when no subpixel vector is
within the constraint. Fix is to not call subpixel motion estimation
when the integer vector is not within the constraint.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar