Bug 719136: Don't attempt to upload when there's nothing to upload. r=jrmuizel

gfx/2d/DrawTargetD2D.cpp

@@ -1985,6 +1985,12 @@ DrawTargetD2D::CreatePartialBitmapForSurface(SourceSurfaceD2D *aSurface, Matrix
   // surface.
   uploadRect = uploadRect.Intersect(rect);
 
+  if (uploadRect.IsEmpty()) {
+    // This bitmap does not cover anything on the screen. XXX -
+    // we need to deal with EXTEND modes here!
+    return NULL;
+  }
+
   if (uploadRect.width <= mRT->GetMaximumBitmapSize() &&
       uploadRect.height <= mRT->GetMaximumBitmapSize()) {