Bug 1453567 - Null-check rootSH in nsDocShell::AddState. r=bz, a=RyanVM

Looks like this accidentally got thrown out in the refactoring.

docshell/base/nsDocShell.cpp

@@ -11854,6 +11854,8 @@ nsDocShell::AddState(JS::Handle<JS::Value> aData, const nsAString& aTitle,
   // call ReplaceEntry so that we notify nsIHistoryListeners that an entry
   // was replaced.
   RefPtr<ChildSHistory> rootSH = GetRootSessionHistory();
+  NS_ENSURE_TRUE(rootSH, NS_ERROR_UNEXPECTED);
+
   if (!aReplace) {
     int32_t curIndex = rootSH->Index();
     if (curIndex > -1) {