Bug 1680258 - Don't try to draw rects with 0 width or height r=emalysz

This prevents an assertion from tripping below.

Differential Revision: https://phabricator.services.mozilla.com/D98476

mozglue/misc/PreXULSkeletonUI.cpp

@@ -520,6 +520,10 @@ void RasterizeColorRect(const ColorRect& colorRect) {
   }
 
   for (const DrawRect& rect : drawRects) {
+    if (rect.height <= 0 || rect.width <= 0) {
+      continue;
+    }
+
     // For rounded rectangles, the first thing we do is draw the top and
     // bottom of the rectangle, with the more complicated logic below. After
     // that we can just draw the vertically centered part of the rect like