secureudpclient - a speculative fix for non-reproducible crash

Not much information in a bug report: QByteArray is protected from negative
sizes and QUdpSocket too. FWIW - add one more check, similar to what
the server counterpart already had.

Fixes: QTBUG-83457
Change-Id: I585fa90e0a258d2257e4fed2f24c52b47548bcbb
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit b283ce1e836ab08e602a11ea255ee3d8e537902e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/network/secureudpclient/association.cpp

@@ -112,6 +112,11 @@ void DtlsAssociation::udpSocketConnected()
 
 void DtlsAssociation::readyRead()
 {
+    if (socket.pendingDatagramSize() <= 0) {
+        emit warningMessage(tr("%1: spurious read notification?").arg(name));
+        return;
+    }
+
     //! [6]
     QByteArray dgram(socket.pendingDatagramSize(), Qt::Uninitialized);
     const qint64 bytesRead = socket.readDatagram(dgram.data(), dgram.size());