Bug 1368015 - Null-check selection in nsCaret::GetFrameAndOffset. r=masayuki

A bit of a wallpaper, but it's silly to crash with a null deref here. If
there's no selection there's nothing to invalidate really.

We already null-check it effectively when aOverrideNode is null.

Differential Revision: https://phabricator.services.mozilla.com/D97960

layout/base/nsCaret.cpp

@@ -376,7 +376,7 @@ nsIFrame* nsCaret::GetFrameAndOffset(Selection* aSelection,
     return nullptr;
   }
 
-  if (!focusNode || !focusNode->IsContent()) {
+  if (!focusNode || !focusNode->IsContent() || !aSelection) {
     return nullptr;
   }