diod: make SYS_setgroups test safer

Problem: on failure, the runtime test of SYS_setgroups replaces diod's
supplementary groups with the test ones.

Pass size=0 to getgroups() when testing the group count.

src/cmd/diod.c

@@ -542,7 +542,7 @@ _test_setgroups (void)
         err_exit ("pthread_create");
     if ((err = pthread_join (t, NULL)))
         err_exit ("pthread_join");
-    if ((n = getgroups (ngroups_max, sg)) < 0)
+    if ((n = getgroups (NULL, 0)) < 0)
         err_exit ("getgroups");
     if (n == 0)
         rc = 1;