Fix memory leaks in REST API

The call to MHD_basic_auth_get_username_password allocates memory for both
the password and the username.

mxs_json_add_relation leaked a reference to a JSON object by using
json_array_append instead of json_array_append_new.

server/core/json_api.cc

@@ -59,7 +59,7 @@ void mxs_json_add_relation(json_t* rel, const char* id, const char* type)
     json_t* obj = json_object();
     json_object_set_new(obj, CN_ID, json_string(id));
     json_object_set_new(obj, CN_TYPE, json_string(type));
-    json_array_append(data, obj);
+    json_array_append_new(data, obj);
 }
 
 static string grab_next_component(string* s)