gstat: Set errno = 0 before strtoul

The strtoul function sets errno on error, but does not clear it on
success; when using strtoul and checking errno (as one should) for
ERANGE / EINVAL afterwards, it's important to zero errno first.

While here, remove a dead store.

Reviewed by:	phk
Fixes:	4fe8c1b67b ("Add error and range checking ... ")
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D59270

usr.sbin/gstat/gstat.c

@@ -440,7 +440,7 @@ main(int argc, char **argv)
 			use_gb();
 			break;
 		case 'I':
-			p = NULL;
+			errno = 0;
 			i = strtoul(optarg, &p, 0);
 			if (p == optarg || errno == EINVAL ||
 			    errno == ERANGE) {