Handle { NULL } argv[] properly in manual arg-reduction

Thanks izabera for reporting this!

whoami.c

@@ -18,7 +18,7 @@ main(int argc, char *argv[])
 	uid_t uid;
 	struct passwd *pw;
 
-	argv0 = argv[0], argc--, argv++;
+	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
 
 	if (argc)
 		usage();