gallium/util: fix returning empty box for rectangle intersection

gallium/util: fix returning empty box for rectangle intersection

These functions deal with inclusive coordinates, hence a 0/0/0/0 rect
returned when there's no intersection doesn't actually represent an empty
rectangle. Hence return 0/-1/0/-1 instead.
This fixes some problems in llvmpipe with empty scissor rects (which up
to now didn't really matter because while the intersect test returned the
wrong result all pixels were scissored away later anyway).