Bug 590403. If we are asked to create a similar surface and our underling surface isn't valid just return null. r=jmuizelaar

gfx/thebes/gfxASurface.cpp

@@ -310,6 +310,10 @@ already_AddRefed<gfxASurface>
 gfxASurface::CreateSimilarSurface(gfxContentType aContent,
                                   const gfxIntSize& aSize)
 {
+    if (!mSurface || !mSurfaceValid) {
+      return nsnull;
+    }
+    
     cairo_surface_t *surface =
         cairo_surface_create_similar(mSurface, cairo_content_t(aContent),
                                      aSize.width, aSize.height);

gfx/thebes/gfxXlibSurface.cpp

@@ -182,6 +182,10 @@ already_AddRefed<gfxASurface>
 gfxXlibSurface::CreateSimilarSurface(gfxContentType aContent,
                                      const gfxIntSize& aSize)
 {
+    if (!mSurface || !mSurfaceValid) {
+      return nsnull;
+    }
+
     if (aContent == CONTENT_COLOR) {
         // cairo_surface_create_similar will use a matching visual if it can.
         // However, systems with 16-bit or indexed default visuals may benefit