Fix assertion (that should have turned the tree orange). (Bug 608458) r=roc a2.0=blocking(on 500141)

layout/base/nsCSSRenderingBorders.cpp

@@ -423,7 +423,8 @@ typedef enum {
 // Given three points, p0, p1, and midPoint, move p1 further in to the
 // rectangle (of which aMidPoint is the center) so that it reaches the
 // closer of the horizontal or vertical lines intersecting the midpoint,
-// while maintaing the slope of the line.  p0 and p1 must be distinct.
+// while maintaing the slope of the line.  If p0 and p1 are the same,
+// just move p1 to midPoint (since there's no slope to maintain).
 // FIXME: Extending only to the midpoint isn't actually sufficient for
 // boxes with asymmetric radii.
 static void
@@ -433,7 +434,7 @@ MaybeMoveToMidPoint(gfxPoint& aP0, gfxPoint& aP1, const gfxPoint& aMidPoint)
 
   if (ps.x == 0.0) {
     if (ps.y == 0.0) {
-      NS_NOTREACHED("points should be different");
+      aP1 = aMidPoint;
     } else {
       aP1.y = aMidPoint.y;
     }