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

Thanks izabera for reporting this!

kill.c

@@ -62,7 +62,8 @@ main(int argc, char *argv[])
 	size_t i;
 	int ret = 0, sig = SIGTERM;
 
-	argv0 = argv[0], argc--, argv++;
+	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
+
 	if (!argc)
 		usage();