res_stasis_mailbox.c: Fix leak of mailbox container.

Change-Id: I7d33c1635713047e7d1597c9d882f7dc006d94b4

res/res_stasis_mailbox.c

@@ -93,6 +93,7 @@ struct ast_json *stasis_app_mailboxes_to_json()
 		}
 	}
 	ao2_iterator_destroy(&iter);
+	ao2_ref(mailboxes, -1);
 
 	return array;
 }