lib/chkname.c: Take NUL byte into account

The _SC_LOGIN_NAME_MAX value includes space for the NUL byte.  The
length of name must smaller than this value to be valid.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Cherry-picked-from: 403a2e3771 ("lib/chkname.c: Take NUL byte into account")
Link: <https://github.com/shadow-maint/shadow/pull/935>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/chkname.c

@@ -80,7 +80,7 @@ bool is_valid_user_name (const char *name)
 	 * User names length are limited by the kernel
 	 */
 	maxlen = sysconf(_SC_LOGIN_NAME_MAX);
-	if (strlen(name) > maxlen)
+	if (strlen(name) >= maxlen)
 		return false;
 
 	return is_valid_name (name);