useradd: fix write_full() return value

write_full() returns -1 on error and useradd was checking another value.

Closes: #1072
Fixes: f45498a ("libmisc/write_full.c: Improve write_full()")

Reported-by: <https://github.com/brown-midas>
Suggested-by: <https://github.com/brown-midas>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>

src/useradd.c

@@ -2042,7 +2042,7 @@ static void lastlog_reset (uid_t uid)
 		return;
 	}
 	if (   (lseek (fd, offset_uid, SEEK_SET) != offset_uid)
-	    || (write_full (fd, &ll, sizeof (ll)) != (ssize_t) sizeof (ll))
+	    || (write_full (fd, &ll, sizeof (ll)) != -1)
 	    || (fsync (fd) != 0)) {
 		fprintf (stderr,
 		         _("%s: failed to reset the lastlog entry of UID %lu: %s\n"),