QmlDebug: Check if packet protocol is still alive after flush()

An QAbstractSocket can disconnect itself on flush(), which we call
after sending any packets. This leads to the packet protocol getting
deleted and in the next iteration of the loop
QmlDebugConnectionPrivate::readyRead() we'd use a null pointer.
Prevent that by checking for null.

Change-Id: Idfdb68a10c3e4deee7b305ff3b028247809090a6
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/qmldebug/qmldebugclient.cpp

@@ -191,7 +191,7 @@ void QmlDebugConnectionPrivate::readyRead()
         emit q->opened();
     }
 
-    while (protocol->packetsAvailable()) {
+    while (protocol && protocol->packetsAvailable()) {
         QPacket pack = protocol->read();
         QString name;
         pack >> name;