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

Thanks izabera for reporting this!

chroot.c

@@ -17,7 +17,7 @@ main(int argc, char *argv[])
 	char *shell[] = { "/bin/sh", "-i", NULL }, *aux, *cmd;
 	int savederrno;
 
-	argv0 = argv[0], argc--, argv++;
+	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
 
 	if (!argc)
 		usage();