WindowServer: Control WSClientConnection's window list teardown better

A lot of things happen in response to window destruction, and some of
them may call into the window's WSClientConnection and ask it to look
through its window list.

If we're right in the middle of tearing down the window list, it's not
a great idea to start iterating over it.

Fixes #386.

Servers/WindowServer/WSClientConnection.cpp

@@ -50,6 +50,10 @@ WSClientConnection::WSClientConnection(CLocalSocket& client_socket, int client_i
 
 WSClientConnection::~WSClientConnection()
 {
+    // NOTE: Move the windows out of 'm_windows' before teardown. This prevents code
+    //       that runs in response to window destruction from trying to iterate over
+    //       a partially destroyed window list.
+    auto windows = move(m_windows);
     s_connections->remove(client_id());
 }