Prevent g_strsplit to be called with NULL.

There might be situations where ref_ids is NULL,
so return an error instead of handing it over to g_strsplit()
which will bail out via assert().

base/nvti.c

@@ -957,7 +957,7 @@ nvti_set_category (nvti_t *n, const gint category)
  *
  * @param ref_text The optional text accompanying all references.
  *
- * @return 0 for success. 1 if n was NULL.
+ * @return 0 for success. 1 if n was NULL, 2 if ref_ids was NULL.
  */
 int
 nvti_add_refs (nvti_t *n, const gchar *type, const gchar *ref_ids,
@@ -968,6 +968,9 @@ nvti_add_refs (nvti_t *n, const gchar *type, const gchar *ref_ids,
   if (!n)
     return (1);
 
+  if (!ref_ids)
+    return (2);
+
   split = g_strsplit (ref_ids, ",", 0);
 
   for (item = split; *item; item++)