Bug 857334 - Fix crash in mozilla::a11y::XULTextFieldAccessible::FrameSelection

Bug 857334 - Fix crash in mozilla::a11y::XULTextFieldAccessible::FrameSelection, r=tbsaunde

accessible/src/xul/XULFormControlAccessible.cpp

@@ -830,6 +830,10 @@ already_AddRefed<nsFrameSelection>
 XULTextFieldAccessible::FrameSelection()
 {
   nsCOMPtr<nsIContent> inputContent(GetInputField());
+  NS_ASSERTION(inputContent, "No input content");
+  if (!inputContent)
+    return nullptr;
+
   nsIFrame* frame = inputContent->GetPrimaryFrame();
   return frame ? frame->GetFrameSelection() : nullptr;
 }