gdk: fix crash if visual is NULL

cairo_xlib_surface_get_visual might return NULL so we need to
check for NULL before trying to use it.

https://bugzilla.gnome.org/show_bug.cgi?id=752520

gdk/x11/gdkglcontext-x11.c

@@ -326,6 +326,9 @@ glx_pixmap_get (cairo_surface_t *surface, guint texture_target)
     None
   };
 
+  if (visual == NULL)
+    return NULL;
+
   with_alpha = cairo_surface_get_content (surface) == CAIRO_CONTENT_COLOR_ALPHA;
 
   y_inverted = FALSE;