QAuthenticator: Reset the authentication challenge

Both Negotiate and NTLM are conditioned on the 'challenge' being empty
when starting the authentication process. So let's reset it when we
start the authentication process.

Fixes: QTBUG-83370
Change-Id: I41af6d5bcfe3dd980ca2bedce10ceff4f61047ff
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/kernel/qauthenticator.cpp

@@ -531,6 +531,7 @@ QByteArray QAuthenticatorPrivate::calculateResponse(const QByteArray &requestMet
                 response = qNtlmPhase3(this, QByteArray::fromBase64(challenge)).toBase64();
                 phase = Done;
             }
+            challenge = "";
         }
 
         break;
@@ -560,6 +561,7 @@ QByteArray QAuthenticatorPrivate::calculateResponse(const QByteArray &requestMet
             if (!phase3Token.isEmpty()) {
                 response = phase3Token.toBase64();
                 phase = Done;
+                challenge = "";
             }
         }