ATS 7.1.3 with bnull-transaform plugin hangs when server aborts a transaction.
Last few log lines before ATS hangs in this state
```
[Sep 17 14:50:53.999] Server {0x2ba8c98be700} DEBUG: <HttpSM.cc:3053
(tunnel_handler_server)> (http) [0] [HttpSM::tunnel_handler_server] aborting
HTTP tunnel due to server truncation
[Sep 17 14:50:53.999] Server {0x2ba8c98be700} DEBUG: <HttpServerSession.cc:125
(do_io_close)> (http_ss) [0] session closing, netvc 0x2ba90c018800
[Sep 17 14:50:53.999] Server {0x2ba8c98be700} DEBUG: <HttpSM.cc:2679
(main_handler)> (http) [0] [HttpSM::main_handler, HTTP_TUNNEL_EVENT_DONE]
[Sep 17 14:50:53.999] Server {0x2ba8c98be700} DEBUG: <HttpSM.cc:1246
(state_response_wait_for_transform_read)> (http) [0]
[&HttpSM::state_response_wait_for_transform_read, HTTP_TUNNEL_EVENT_DONE]
[Sep 17 14:50:53.999] Server {0x2ba8c98be700} DEBUG: <HttpSM.cc:1276
(state_common_wait_for_transform_read)> (http) [0]
[&HttpSM::state_common_wait_for_transform_read, HTTP_TUNNEL_EVENT_DONE]
```
[ Full content available at:
https://github.com/apache/trafficserver/issues/4251 ]
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