Http: don't try to send request on a closed socket

We have code in the protocolHandlers that tries to handle this case, but
if we have an error before we create protocolHandler (read: proxy
complains about something) we will assert in debug, or
deref nullptr in release.

Pick-to: 6.6 6.5 6.2
Change-Id: I4bde9c8af0fa96dc11f77ca4d4b5cb84c31b54fa
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/qhttpnetworkconnection.cpp

@@ -1046,6 +1046,11 @@ void QHttpNetworkConnectionPrivate::_q_startNextRequest()
     //resend the necessary ones.
     for (int i = 0; i < activeChannelCount; ++i) {
         if (channels[i].resendCurrent && (channels[i].state != QHttpNetworkConnectionChannel::ClosingState)) {
+            if (!channels[i].socket
+                || channels[i].socket->state() == QAbstractSocket::UnconnectedState) {
+                if (!channels[i].ensureConnection())
+                    continue;
+            }
             channels[i].resendCurrent = false;
 
             // if this is not possible, error will be emitted and connection terminated