QAbstractSocket: clear error code to match the error description

Since 5.10, QIODevice resets the error string on opening. So, we should
set the error code to UnknownSocketError accordingly.

Change-Id: I0dd314788ffc182d6837f9d06b51e41d6de59d7e
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/socket/qabstractsocket.cpp

@@ -1694,6 +1694,8 @@ void QAbstractSocket::connectToHost(const QString &hostName, quint16 port,
     }
 #endif
 
+    // Sync up with error string, which open() shall clear.
+    d->socketError = UnknownSocketError;
     if (openMode & QIODevice::Unbuffered)
         d->isBuffered = false;
     else if (!d_func()->isBuffered)
@@ -1929,6 +1931,8 @@ bool QAbstractSocket::setSocketDescriptor(qintptr socketDescriptor, SocketState
         return false;
     }
 
+    // Sync up with error string, which open() shall clear.
+    d->socketError = UnknownSocketError;
     if (d->threadData->hasEventDispatcher())
         d->socketEngine->setReceiver(d);
 

src/network/socket/qabstractsocket_p.h

@@ -153,6 +153,7 @@ public:
     QAbstractSocket::SocketType socketType;
     QAbstractSocket::SocketState state;
 
+    // Must be kept in sync with QIODevicePrivate::errorString.
     QAbstractSocket::SocketError socketError;
 
     QAbstractSocket::NetworkLayerProtocol preferredNetworkLayerProtocol;