login & su: Treat an empty passwd field as invalid (#315)

* login & su: Treat an empty passwd field as invalid

Otherwise it's treated like the “require no password” clause while it probably
should be treated like a normal su that can't validate anyway.

A similar change should be done for USE_PAM.

* su & login: Introduce PREVENT_NO_AUTH

lib/getdef.c

@@ -164,6 +164,7 @@ static struct itemdef def_table[] = {
 #endif
 	{"FORCE_SHADOW", NULL},
 	{"GRANT_AUX_GROUP_SUBIDS", NULL},
+	{"PREVENT_NO_AUTH", NULL},
 	{NULL, NULL}
 };