tst_qnetworkreply: Fix divide by zero crash on MacOS X

Change-Id: Ie1b6d5aa25a745eb6c16041151141efe99c051aa
Reviewed-on: http://codereview.qt.nokia.com/422
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>

tests/auto/qnetworkreply/tst_qnetworkreply.cpp

@@ -5970,7 +5970,7 @@ public slots:
 
     void onReadAndReschedule() {
         const qint64 bytesReceived = m_reply->bytesAvailable();
-        if (bytesReceived) {
+        if (bytesReceived && m_reply->readBufferSize()) {
            QByteArray data = m_reply->read(bytesReceived);
            // reschedule read
            const int millisecDelay = static_cast<int>(bytesReceived * 1000 / m_reply->readBufferSize());