When GetCredential() callback is not set, don't use authentications requiring it.

The auth methods that employ Getcredential() will only be used if the client's
GetCredential callback is actually set.

libvncclient/rfbproto.c

@@ -569,8 +569,8 @@ ReadSupportedSecurityType(rfbClient* client, uint32_t *result, rfbBool subAuth)
         rfbClientLog("%d) Received security type %d\n", loop, tAuth[loop]);
         if (flag) continue;
         if (tAuth[loop]==rfbVncAuth || tAuth[loop]==rfbNoAuth ||
-            tAuth[loop]==rfbARD ||
-            (!subAuth && (tAuth[loop]==rfbTLS || tAuth[loop]==rfbVeNCrypt)))
+            (tAuth[loop]==rfbARD && client->GetCredential) ||
+            (!subAuth && (tAuth[loop]==rfbTLS || (tAuth[loop]==rfbVeNCrypt && client->GetCredential))))
         {
             if (!subAuth && client->clientAuthSchemes)
             {