WindowServer: Close all menus belonging to a client when it disconnects

Previously we would be left with a menu stack containing nulled-out
WeakPtr's to menus in the now-disconnected clients.

This was tripping up an assertion when clicking anywhere after shutting
down a program while it had a menu open.

Servers/WindowServer/WSClientConnection.cpp

@@ -49,6 +49,7 @@ WSClientConnection::WSClientConnection(CLocalSocket& client_socket, int client_i
 
 WSClientConnection::~WSClientConnection()
 {
+    WSWindowManager::the().menu_manager().close_all_menus_from_client({}, *this);
     auto windows = move(m_windows);
 }