Platform: cleanup X11 stuff only if it's there.

Of course, widely expepted assumptions DO NOT work here. It doesn't
even report an error, it crashes straight away.

src/Magnum/Platform/AbstractXApplication.cpp

@@ -114,8 +114,8 @@ AbstractXApplication::~AbstractXApplication() {
     _contextHandler.reset();
 
     /* Shut down X */
-    XDestroyWindow(_display, _window);
-    XCloseDisplay(_display);
+    if(_window) XDestroyWindow(_display, _window);
+    if(_display) XCloseDisplay(_display);
 }
 
 void AbstractXApplication::swapBuffers() {