QNetworkReplyHttpImpl - check 'isOpen' twice

A signal emitted after the first 'isOpen' check on QNetworkReply can
trigger a slot that aborts/closes our reply. So before we can append
any data to reply's buffer, we have to re-check that we're still open.

Task-number: QTBUG-59909
Change-Id: I781d5a4fd5fc30d485af63f45cf36c254378af64
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/qnetworkreplyhttpimpl.cpp

@@ -1883,6 +1883,12 @@ void QNetworkReplyHttpImplPrivate::_q_cacheLoadReadyRead()
                                      totalSize.isNull() ? Q_INT64_C(-1) : totalSize.toLongLong());
         }
     }
+
+    // A signal we've emitted might be handled by a slot that aborts,
+    // so we need to check for that and bail out if it's happened:
+    if (!q->isOpen())
+        return;
+
     // If there are still bytes available in the cacheLoadDevice then the user did not read
     // in response to the readyRead() signal. This means we have to load from the cacheLoadDevice
     // and buffer that stuff. This is needed to be able to properly emit finished() later.