RoomList: do not blindly dereference RoomState (#232)

This can happen when a new room is being created from a different client and thus is created
with no known state to the local client.

src/RoomList.cc

@@ -199,12 +199,15 @@ RoomList::sync(const std::map<QString, QSharedPointer<RoomState>> &states,
                         addRoom(settings.at(state.first), state.second, state.first);
 
                 auto room       = rooms_[state.first];
+                auto room_state = room->state();
 
-                auto current_avatar = room->state()->getAvatar();
+                if (room_state) {
+                        auto current_avatar = room_state->getAvatar();
                         auto new_avatar     = state.second->getAvatar();
 
                         if (current_avatar != new_avatar && !new_avatar.toString().isEmpty())
                                 updateAvatar(state.first, new_avatar.toString());
+                }
 
                 room->setState(state.second);
         }