ldap: silence clang warning

Use memset() to initialize a structure to avoid LLVM/Clang warning:
ldap.c:193:39: warning: missing field 'UserLength' initializer [-Wmissing-field-initializers]

Closes https://github.com/curl/curl/pull/1992

lib/ldap.c

@@ -190,9 +190,11 @@ static int ldap_win_bind_auth(LDAP *server, const char *user,
                               const char *passwd, unsigned long authflags)
 {
   ULONG method = 0;
-  SEC_WINNT_AUTH_IDENTITY cred = { 0, };
+  SEC_WINNT_AUTH_IDENTITY cred;
   int rc = LDAP_AUTH_METHOD_NOT_SUPPORTED;
 
+  memset(&cred, 0, sizeof(cred));
+
 #if defined(USE_SPNEGO)
   if(authflags & CURLAUTH_NEGOTIATE) {
     method = LDAP_AUTH_NEGOTIATE;