PollHandler: Ensure that m_pollStartEvent is always initialized to null

Otherwise it may be undefined, and we DO create default-constructed
PollHandler. For example, one is used as a fallback poll object
in NeoChatRoom::poll.

This is blind fix for a pretty nasty poll-related crash we saw a few
months ago.

BUG: 493649

src/pollhandler.h

@@ -91,7 +91,7 @@ Q_SIGNALS:
     void hasEndedChanged();
 
 private:
-    const Quotient::PollStartEvent *m_pollStartEvent;
+    const Quotient::PollStartEvent *m_pollStartEvent = nullptr;
 
     void updatePoll(Quotient::RoomEventsRange events);