Reset TQThreadInstance::currentThread() value when destroying main thread

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>

src/kernel/qthread_unix.cpp

@@ -183,6 +183,10 @@ void TQThreadInstance::finish( void * )
 void TQThreadInstance::finishGuiThread(TQThreadInstance *d) {
     TQThreadStorageData::finish( d->thread_storage );
     d->thread_storage = 0;
+
+    // You should use TQApplication::currentThread() to check if GUI thread wasn't yet destroyed,
+    // but just in case we better cleanup the pointer here as well.
+    setCurrentThread(0);
 }
 
 TQMutex *TQThreadInstance::mutex() const