Debugger: Fix Segmentation Fault on Windows

This amends 035b8aa5f7. shutdown() is
called from GdbEngine and from OutputColloector's Dtor. Thus there is a
change we access m_socket, after it is nullified.

Change-Id: I7d26ded70d0b148d83eb551edaf9b82850db197e
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/outputcollector.cpp

@@ -156,6 +156,7 @@ void OutputCollector::newConnectionAvailable()
 void OutputCollector::bytesAvailable()
 {
 #ifdef Q_OS_WIN
+    if (m_socket)
         emit byteDelivery(m_socket->readAll());
 #else
     size_t nbytes = 0;