loopback: Don't free credentials borrowed from the connection

We currently get away with this because the connection isn't fully
freed before we exit, but the connection is meant to own the result
of _dbus_connection_get_credentials() (it's "(transfer none)" in
GLib terminology).

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107194

test/loopback.c

@@ -302,7 +302,6 @@ test_connect (Fixture *f,
                   _dbus_string_get_const_data (&cred_string));
   g_assert_cmpstr (_dbus_string_get_const_data (&cred_string), !=, NULL);
   _dbus_string_free (&cred_string);
-  _dbus_clear_credentials (&creds);
 
   dbus_free (address);
 }