BTSession.close: improve close handler

After bluetooth socket close, set None to the socket reference to ensure
no use.

This reverts commit 7fcaba55c4e700142310cefa00e63fd462bff6e2.

scratch_link.py

@@ -263,6 +263,7 @@ class BTSession(Session):
         if self.sock:
             logger.info(f"disconnect to BT socket: {self.sock}")
             self.sock.close()
+            self.sock = None
 
     def __del__(self):
         self.close()