fix single-threaded mtag

copyparty/up2k.py

@@ -552,6 +552,7 @@ class Up2k(object):
                     last_write = time.time()
                     n_buf = 0
 
+            if mpool:
                 self._stop_mpool(mpool)
                 with self.mutex:
                     n_add += len(self._flush_mpool(c3))