Fixed CThreadRep to not raise a signal on the thread if it's

already dead.  Otherwise the signal could propagate to the
parent thread (at least on linux threads) and cause havoc.

lib/mt/CThreadRep.cpp

@@ -389,7 +389,7 @@ void
 CThreadRep::cancel()
 {
 	CLock lock(s_mutex);
-	if (m_cancellable && !m_cancelling) {
+	if (!m_exit && m_cancellable && !m_cancelling) {
 		m_cancel = true;
 	}
 	else {