Fix a crash when grabbing relationAuthor

There's a bug in how we're using this function in room search, but we
definitely don't want it to crash. The event is technically not in the
timeline, so we were dereferencing an invalid iterator or whatever.

src/libneochat/chatbarcache.cpp

@@ -150,7 +150,12 @@ Quotient::RoomMember ChatBarCache::relationAuthor() const
     if (m_relationId.isEmpty()) {
         return room->member(QString());
     }
-    return room->member((*room->findInTimeline(m_relationId))->senderId());
+    const auto evtIt = room->findInTimeline(m_relationId);
+    if (evtIt != room->messageEvents().rend()) {
+        return room->member((*evtIt)->senderId());
+    }
+    qWarning() << "Failed to find relation" << m_relationId << "in timeline?";
+    return room->member(QString());
 }
 
 bool ChatBarCache::relationAuthorIsPresent() const