device: Fix dispose

The way master devices detach from their other master counterpart is
vulnerable to infinite recursion due to the way we first recurse on
the other device before clearing the pointer, this may happen if
that last reference to the other master device is held by the
device->associated field.

https://bugzilla.gnome.org/show_bug.cgi?id=732742

gdk/gdkdevice.c

@@ -323,17 +323,20 @@ static void
 gdk_device_dispose (GObject *object)
 {
   GdkDevice *device = GDK_DEVICE (object);
+  GdkDevice *associated = device->associated;
 
-  if (device->type == GDK_DEVICE_TYPE_SLAVE)
-    _gdk_device_remove_slave (device->associated, device);
+  if (associated && device->type == GDK_DEVICE_TYPE_SLAVE)
+    _gdk_device_remove_slave (associated, device);
 
-  if (device->associated)
+  if (associated)
     {
-      if (device->type == GDK_DEVICE_TYPE_MASTER)
-        _gdk_device_set_associated_device (device->associated, NULL);
-
-      g_object_unref (device->associated);
       device->associated = NULL;
+
+      if (device->type == GDK_DEVICE_TYPE_MASTER &&
+          associated->associated == device)
+        _gdk_device_set_associated_device (associated, NULL);
+
+      g_object_unref (associated);
     }
 
   G_OBJECT_CLASS (gdk_device_parent_class)->dispose (object);