Fix some special cases of using mu_app_init.

* libmu_argp/muinit.c (mu_app_init): Allow capa==NULL.
* mailbox/cfg_driver.c (mu_config_clone_container): Do nothing
if cont == NULL.

@@ -68,6 +68,7 @@ mu_app_init (struct argp *myargp, const char **capa,
   
   mu_set_program_name (argv[0]);
   mu_libargp_init ();
+  if (capa)
     for (i = 0; capa[i]; i++)
       mu_gocs_register_std (capa[i]); /*FIXME*/
   if (!myargp)

@@ -320,6 +320,8 @@ _clone_action (void *item, void *cbdata)
 int
 mu_config_clone_container (struct mu_cfg_cont *cont)
 {
+  if (!cont)
+    return 0;
   mu_refcount_inc (cont->refcount);
   /* printf("clone %p-%s: %d\n", cont, cont->v.section.ident, n); */
   switch (cont->type)