[recipes] Avoid a crash

[Matthias Clasen <matthiasc src gnome org>]

commit c48bca0a460c7e424ef5b534ffab51e2146b5f33
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Feb 13 17:49:35 2017 -0500

    Avoid a crash
    
    We pass NULL for callback in some places, so better check first.

 src/gr-account.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/gr-account.c b/src/gr-account.c
index 6cf1695..d905b8c 100644
--- a/src/gr-account.c
+++ b/src/gr-account.c
@@ -314,7 +314,8 @@ gr_ensure_user_chef (GtkWindow        *window,
 
         chef = gr_recipe_store_get_chef (store, gr_recipe_store_get_user_key (store));
         if (chef) {
-                callback (chef, data);
+                if (callback)
+                        callback (chef, data);
                 return;
         }