Fix crash when trying to view Security settings in an invited room

This state event doesn't exist (or is inaccessible) to us, and tries to
access nullptr.

(cherry picked from commit 268975bc3b)

src/neochatroom.cpp

@@ -747,7 +747,10 @@ QList<QString> NeoChatRoom::restrictedIds() const
 
 QString NeoChatRoom::historyVisibility() const
 {
-    return currentState().get("m.room.history_visibility"_ls)->contentJson()["history_visibility"_ls].toString();
+    if (auto stateEvent = currentState().get("m.room.history_visibility"_ls)) {
+        return stateEvent->contentJson()["history_visibility"_ls].toString();
+    }
+    return {};
 }
 
 void NeoChatRoom::setHistoryVisibility(const QString &historyVisibilityRule)