GdkWindow: fix conditional jump depends on uninitialized value error

Those if() blocks don't have any reason being there, as x and y are not
pointers. If the window is destroyed, just set the out values to zero
and return.

As seen in valgrind:

==3306== Conditional jump or move depends on uninitialised value(s)
==3306==    at 0x624C74F: gdk_window_get_root_coords (gdkwindow.c:6933)
==3306==    by 0x5E193C3: gtk_tooltip_show_tooltip (gtktooltip.c:1160)
==3306==    by 0x5E19C05: tooltip_popup_timeout (gtktooltip.c:1282)
==3306==    by 0x623B102: gdk_threads_dispatch (gdk.c:754)
==3306==    by 0x8592F3A: g_timeout_dispatch (gmain.c:3907)
==3306==    by 0x859174C: g_main_context_dispatch (gmain.c:2441)
==3306==    by 0x8591F47: g_main_context_iterate (gmain.c:3089)
==3306==    by 0x8592494: g_main_loop_run (gmain.c:3297)
==3306==    by 0x5D2E501: gtk_main (gtkmain.c:1362)
==3306==    by 0x5C5652F: gtk_application_run_mainloop
(gtkapplication.c:115)
==3306==    by 0x7C47C9D: g_application_run (gapplication.c:1323)
==3306==    by 0x447B5F: main (nautilus-main.c:102)
==3306==  Uninitialised value was created by a stack allocation
==3306==    at 0x624D48A: gdk_window_get_device_position
(gdkwindow.c:4952)

gdk/gdkwindow.c

@@ -6930,10 +6930,8 @@ gdk_window_get_root_coords (GdkWindow *window,
 
   if (GDK_WINDOW_DESTROYED (window))
     {
-      if (x)
-	*root_x = x;
-      if (y)
-	*root_y = y;
+      *root_x = 0;
+      *root_y = 0;
       return;
     }