Debugger: Fix cdb switching back from stopper thread

Amends 9f9c72302f.

Change-Id: If97ca13de30fa4d24deddd58177f55e2c71010a8
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/threadshandler.cpp

@@ -285,6 +285,7 @@ Thread ThreadsHandler::currentThread() const
 
 void ThreadsHandler::setCurrentThread(const Thread &thread)
 {
+    QTC_ASSERT(thread, return);
     if (thread == m_currentThread)
         return;