gears.geometry: Fix rectangle intersection computation

Previously, when gears.geometry.rectangle.get_intersection() was called
with two non-intersecting rectangles, it would return a negative
width/height.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/geometry.lua

@@ -176,6 +176,9 @@ function gears.geometry.rectangle.get_intersection(a, b)
     g.y = math.max(a.y, b.y)
     g.width = math.min(a.x + a.width, b.x + b.width) - g.x
     g.height = math.min(a.y + a.height, b.y + b.height) - g.y
+    if g.width <= 0 or g.height <= 0 then
+        g.width, g.height = 0, 0
+    end
     return g
 end