Fix memory access violation for Empty/payload-less messages

as per https://tools.ietf.org/html/rfc7252#section-3

   Following the header, token, and options, if any, comes the optional
   payload.  If present and of non-zero length, it is prefixed by a
   fixed, one-byte Payload Marker (0xFF), which indicates the end of
   options and the start of the payload.

Change-Id: Ib3ce645970b8b78fa055a5807e12762220509427
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/coap/qcoapinternalreply.cpp

@@ -123,7 +123,7 @@ QCoapInternalReply *QCoapInternalReply::createFromFrame(const QByteArray &reply,
     }
 
     // Parse Payload
-    if (pduData[i] == 0xFF) {
+    if (i < reply.length() && pduData[i] == 0xFF) {
         // -1 because of 0xFF at the beginning
         QByteArray currentPayload = reply.mid(i + 1);
         d->message.setPayload(d->message.payload().append(currentPayload));