wincred: handle empty username/password correctly

Empty (length 0) usernames and/or passwords, when saved in the Windows
Credential Manager, come back as null when reading the credential.

One use case for such empty credentials is with NTLM authentication, where
empty username and password instruct libcurl to authenticate using the
credentials of the currently logged-on user (single sign-on).

When locating the relevant credentials, make empty username match null.
When outputting the credentials, handle nulls correctly.

Signed-off-by: Jakub Bereżański <kuba@berezanscy.pl>

Author: jberezanski

contrib/credential/wincred/git-credential-wincred.c

@@ -94,6 +94,12 @@ static WCHAR *wusername, *password, *protocol, *host, *path, target[1024];
 static void write_item(const char *what, LPCWSTR wbuf, int wlen)
 {
 	char *buf;
+
+	if (!wbuf || !wlen) {
+		printf("%s=\n", what);
+		return;
+	}
+
 	int len = WideCharToMultiByte(CP_UTF8, 0, wbuf, wlen, NULL, 0, NULL,
 	    FALSE);
 	buf = xmalloc(len);
@@ -141,7 +147,7 @@ static int match_part(LPCWSTR *ptarget, LPCWSTR want, LPCWSTR delim)
 static int match_cred(const CREDENTIALW *cred)
 {
 	LPCWSTR target = cred->TargetName;
-	if (wusername && wcscmp(wusername, cred->UserName))
+	if (wusername && wcscmp(wusername, cred->UserName ? cred->UserName : L""))
 		return 0;
 
 	return match_part(&target, L"git", L":") &&
@@ -164,7 +170,7 @@ static void get_credential(void)
 	for (i = 0; i < num_creds; ++i)
 		if (match_cred(creds[i])) {
 			write_item("username", creds[i]->UserName,
-				wcslen(creds[i]->UserName));
+				creds[i]->UserName ? wcslen(creds[i]->UserName) : 0);
 			write_item("password",
 				(LPCWSTR)creds[i]->CredentialBlob,
 				creds[i]->CredentialBlobSize / sizeof(WCHAR));