Issue #1515 - Add null check to nsCSSFrameConstructor::IsValidSibling

With the changes to layout for WebComponents, it is now apparently possible to
pass in null for frame tree items to this function, which would cause a null
deref crash if not checked.

layout/base/nsCSSFrameConstructor.cpp

@@ -6464,6 +6464,11 @@ nsCSSFrameConstructor::IsValidSibling(nsIFrame*              aSibling,
                                       nsIContent*            aContent,
                                       StyleDisplay&          aDisplay)
 {
+  if (!aSibling || !aContent) {
+    // If either of these are null, no sane comparison can be made.
+    return false;
+  }
+  
   nsIFrame* parentFrame = aSibling->GetParent();
   nsIAtom* parentType = parentFrame->GetType();