test-cgroup-mask: check return value

CID #1339830.

src/test/test-cgroup-mask.c

@@ -40,6 +40,7 @@ static int test_cgroup_mask(void) {
                 puts("manager_new: Permission denied. Skipping test.");
                 return EXIT_TEST_SKIP;
         }
+        assert_se(r >= 0);
 
         /* Turn off all kinds of default accouning, so that we can
          * verify the masks resulting of our configuration and nothing