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

Thanks izabera for reporting this!

sponge.c

@@ -17,7 +17,7 @@ main(int argc, char *argv[])
 	char tmp[] = "/tmp/sponge-XXXXXX";
 	int fd, tmpfd;
 
-	argv0 = argv[0], argc--, argv++;
+	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
 
 	if (argc != 1)
 		usage();