[gegl] buffer: avoid a potential NULL dereference

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 81b6a8493feb05c49999d9c7f441e73ef05e87f8
Author: Øyvind Kolås <pippin gimp org>
Date:   Wed Jun 10 04:35:58 2020 +0200

    buffer: avoid a potential NULL dereference

 gegl/buffer/gegl-buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gegl/buffer/gegl-buffer.c b/gegl/buffer/gegl-buffer.c
index 6c6926ebd..ed1a1b5be 100644
--- a/gegl/buffer/gegl-buffer.c
+++ b/gegl/buffer/gegl-buffer.c
@@ -1081,7 +1081,7 @@ gegl_buffer_create_sub_buffer (GeglBuffer          *buffer,
   if (extent == NULL)
     extent = gegl_buffer_get_extent (buffer);
 
-  if (extent->width < 0 || extent->height < 0)
+  if (!extent || extent->width < 0 || extent->height < 0)
     {
       g_warning ("avoiding creating buffer of size: %ix%i returning an empty buffer instead.\n", 
extent->width, extent->height);
       return g_object_new (GEGL_TYPE_BUFFER,