x11: Be a lot more careful about setting ParentRelative

We don't want to set ParentRelative when:

- the parent window is NULL
  In that case we are unsure about the depth, so better err on the side
  of caution and avoid a BadMatch by accepting ugly output.
- the cairo pattern is in an error status
  This should never happen - unless you start up in OOM - but better
  be safe than sorry.

Might help with the spurious crashes in #1280.

gdk/x11/gdkwindow-x11.c

@@ -3005,7 +3005,8 @@ G_GNUC_END_IGNORE_DEPRECATIONS
       /* X throws BadMatch if the parent has a different depth when
        * using ParentRelative */
       parent = gdk_window_get_parent (window);
-      if (parent == NULL || window->depth == parent->depth)
+      if (parent != NULL && window->depth == parent->depth &&
+          cairo_pattern_status (pattern) == CAIRO_STATUS_SUCCESS)
         {
           XSetWindowBackgroundPixmap (GDK_WINDOW_XDISPLAY (window),
                                       GDK_WINDOW_XID (window), ParentRelative);