window: Only draw resize grip if its window exists

window: Only draw resize grip if its window exists

Don't rely on priv->resize_grip_visible as the code comment in the
variable declaration indicates.
This fixes warnings with GtkPlug, which can cause resize_grip_visible to
be TRUE but grid_window to be NULL - running tests/teststatusicon
reproduces this.

This broke with 7ef113ce56a75641175af31267e2075f634267e0