Network-chat example: Only disconnect if still connected

If we are already disconnected we end up printing warnings about how it's
illegal to call waitForBytesWritten on an unconnected socket

Change-Id: I8c864d105d5372edfee84affb362cca3021b8ea0
Reviewed-by: Konrad Kujawa <konrad.kujawa@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

examples/network/network-chat/connection.cpp

@@ -48,7 +48,7 @@ Connection::Connection(qintptr socketDescriptor, QObject *parent)
 
 Connection::~Connection()
 {
-    if (isGreetingMessageSent) {
+    if (isGreetingMessageSent && QAbstractSocket::state() != QAbstractSocket::UnconnectedState) {
         // Indicate clean shutdown.
         writer.endArray();
         waitForBytesWritten(2000);