logind: fail user creation is slice creation fails

If the slice wasn't set, logind would core dump later in the process:
systemd-logind[2672]: Assertion 's->user->slice' failed at ../src/login/logind-session.c:531, function session_start_scope().

https://bugzilla.redhat.com/show_bug.cgi?id=1182781

src/login/logind-user.c

@@ -351,6 +351,7 @@ static int user_start_slice(User *u) {
                         dbus_error_free(&error);
 
                         free(slice);
+                        return r;
                 } else {
                         u->slice = slice;