xff: HttpXFFGetCfg - allow conf to be NULL

The code fully handles conf being NULL, and we have other functions
where conf can be NULL.

src/app-layer-htp-xff.c

@@ -201,7 +201,7 @@ end:
  */
 void HttpXFFGetCfg(ConfNode *conf, HttpXFFCfg *result)
 {
-    BUG_ON(conf == NULL || result == NULL);
+    BUG_ON(result == NULL);
 
     ConfNode *xff_node = NULL;