[seed] seed_script_destroy: only unref source_url if it exists

[Alan Knowles <alank src gnome org>]

commit 96e375f91cdeb48dc4d5da7f24d4ce0f8a1b54e9
Author: Emilio Pozuelo Monfort <emilio pozuelo collabora co uk>
Date:   Wed Jun 12 11:10:12 2013 +0200

    seed_script_destroy: only unref source_url if it exists
    
    Fixes a crash when creating a script with a NULL source_url,
    which is documented to be fine.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=702082

 libseed/seed-api.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/libseed/seed-api.c b/libseed/seed-api.c
index ced6704..79d61cb 100644
--- a/libseed/seed-api.c
+++ b/libseed/seed-api.c
@@ -415,7 +415,8 @@ void
 seed_script_destroy (SeedScript * s)
 {
   seed_string_unref (s->script);
-  seed_string_unref (s->source_url);
+  if (s->source_url)
+    seed_string_unref (s->source_url);
 
   g_free (s);
 }