gnome-desktop-thumbnail.c: Initialize empty pixbuf.

ref:
https://gitlab.gnome.org/GNOME/gnome-desktop/-/commit/911091d76eef79f866f2faaec7eae5df54ce769f

Author: mtwebster

libcinnamon-desktop/gnome-desktop-thumbnail.c

@@ -1627,6 +1627,8 @@ gnome_desktop_thumbnail_factory_create_failed_thumbnail (GnomeDesktopThumbnailFa
 
   g_snprintf (mtime_str, 21, "%ld",  mtime);
   pixbuf = gdk_pixbuf_new (GDK_COLORSPACE_RGB, TRUE, 8, 1, 1);
+  gdk_pixbuf_fill (pixbuf, 0x00000000);
+
   saved_ok  = gdk_pixbuf_save (pixbuf,
 			       tmp_path,
 			       "png", NULL,