Fix OCSP-Stapling error if identity cannot be verified

If QSslConfiguration::setCaCertificates is set to empty list openssl
cannot verify the OCSP-Response.
Qt will provide it as QSslError::OcspResponseCannotBeTrusted that can be
ignored. But the openssl error is still in the error queue and prevents
a successful reply in QNetworkReply::finished.

So let's clear the queue after OCSP checking to avoid side affects.

Change-Id: I44a7f45a2eebd20ea86a235a0534f80986c40a26
Fixes: QTBUG-85638
Pick-to: 5.15
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/ssl/qsslsocket_openssl.cpp

@@ -84,6 +84,7 @@
 #include <QtCore/qurl.h>
 #include <QtCore/qvarlengtharray.h>
 #include <QtCore/qscopedvaluerollback.h>
+#include <QtCore/qscopeguard.h>
 #include <QtCore/qlibrary.h>
 #include <QtCore/qoperatingsystemversion.h>
 
@@ -1802,6 +1803,10 @@ bool QSslSocketBackendPrivate::checkOcspStatus()
     Q_ASSERT(mode == QSslSocket::SslClientMode); // See initSslContext() for SslServerMode
     Q_ASSERT(configuration.peerVerifyMode != QSslSocket::VerifyNone);
 
+    const auto clearErrorQueue = qScopeGuard([] {
+        logAndClearErrorQueue();
+    });
+
     ocspResponses.clear();
     ocspErrorDescription.clear();
     ocspErrors.clear();