[gegl] gegl: only set error message when error destptr is passed

[Øyvind Kolås <ok src gnome org>]

commit 11da352e21655d320fca8d7cc3fdf15feca9f921
Author: Øyvind Kolås <pippin gimp org>
Date:   Mon Nov 27 15:50:51 2017 +0100

    gegl: only set error message when error destptr is passed

 gegl/gegl-serialize.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gegl/gegl-serialize.c b/gegl/gegl-serialize.c
index 23b69a4..6db8c5e 100644
--- a/gegl/gegl-serialize.c
+++ b/gegl/gegl-serialize.c
@@ -492,7 +492,7 @@ gegl_create_chain_argv (char      **ops,
 
                         if (value[0] && babl_format_exists (value))
                           format = babl_format (value);
-                        else
+                        else if (error)
                           {
                             char *error_str = g_strdup_printf (
                                   "BablFormat \"%s\" doest not exist.", value);