filechooser: Don't crash if invalid thumbnails exist

Failing to load a thumbnail returns a NULL pixbuf. Since the hidpi
patches this wasn't checked when creating the surface. Result: assertion
failure.

https://bugzilla.gnome.org/show_bug.cgi?id=719977

gtk/gtkfilesystem.c

@@ -813,10 +813,13 @@ _gtk_file_info_render_icon (GFileInfo *info,
 						 icon_size*scale, icon_size*scale,
 						 NULL);
 
+      if (pixbuf != NULL)
+        {
           surface = gdk_cairo_surface_create_from_pixbuf (pixbuf, scale,
                                                           gtk_widget_get_window (widget));
           g_object_unref (pixbuf);
         }
+    }
 
   if (!surface)
     {