Don't dispose shadow server before all clients have disconnected

Ensure all client threads are disconnected before we stop the server
thread and thus make it available for deletion. We do this because
shadow_client_context_free() accesses rdpShadowServer structure, which
we have to keep alive as long as it is needed.

Calling shadow_client_context_free() after shadow_server_stop() and
shadow_server_free() were called may have caused an application crash.

server/shadow/shadow_server.c

@@ -344,6 +344,14 @@ void* shadow_server_thread(rdpShadowServer* server)
 
 	shadow_subsystem_stop(server->subsystem);
 
+	/* Signal to the clients that server is being stopped and wait for them
+	 * to disconnect. */
+	MessageQueue_PostQuit(subsystem->MsgPipe->Out, 0);
+	while(ArrayList_Count(server->clients) > 0)
+	{
+		Sleep(100);
+	}
+
 	ExitThread(0);
 
 	return NULL;