credui: Sign-compare warnings fix

[Andrew Talbot]

Changelog:
    credui: Sign-compare warnings fix.

diff --git a/dlls/credui/credui_main.c b/dlls/credui/credui_main.c
index 42316d6..7e3c8a0 100644
--- a/dlls/credui/credui_main.c
+++ b/dlls/credui/credui_main.c
@@ -441,7 +441,7 @@ static void CredDialogCommandOk(HWND hwndDlg, struct cred_dialog_params *params)
 
     if (!strchrW(user, '\\') && !strchrW(user, '@'))
     {
-        INT len_target = strlenW(params->pszTargetName);
+        ULONG len_target = strlenW(params->pszTargetName);
         memcpy(params->pszUsername, params->pszTargetName,
                min(len_target, params->ulUsernameMaxChars) * sizeof(WCHAR));
         if (len_target + 1 < params->ulUsernameMaxChars)