Unset pixel buffer when x0vncserver client disconnects.

In XDesktop::start() we allocate pixel buffer and set it as the backend to the given VNCServer.
In XDesktop::stop() we deallocate the buffer, so we must unset it from the VNCServer as well.
Otherwise the VNCServer could try to access it and crash, for example in deferred update.

unix/x0vncserver/XDesktop.cxx

@@ -166,7 +166,8 @@ XDesktop::XDesktop(Display* dpy_, Geometry *geometry_)
 }
 
 XDesktop::~XDesktop() {
-    stop();
+    if (running)
+      stop();
 }
 
 
@@ -227,6 +228,9 @@ void XDesktop::stop() {
       XDamageDestroy(dpy, damage);
 #endif
 
+    server->setPixelBuffer(0);
+    server = 0;
+
     delete pb;
     pb = 0;
 }