bug#15075: 24.3.50; Infinite loop when closing some connections

[Lars Magne Ingebrigtsen]

With an -O0 build, it's almost completely repeatable.

Here's the accompanying Lisp backtrace.  So it seems like we're sending
via gnutls, we send the quit command to the SMTP server, it says "bye"
and closes the connection, and then the next (?) call to
`accept-process-output' infloops.

Lisp Backtrace:
"accept-process-output" (0xffff9db0)
"byte-code" (0xffffa1b0)
"smtpmail-read-response" (0xffffa900)
"byte-code" (0xffffacf0)
"smtpmail-via-smtp" (0xffffb420)
"smtpmail-send-it" (0xffffb910)
"message-smtpmail-send-it" (0xffffbdd0)
"message-multi-smtp-send-mail" (0xffffc2a0)
"gnus-agent-send-mail" (0xffffc770)
"message-send-mail" (0xffffcc70)
"message-send-via-mail" (0xffffd130)
"message-send" (0xffffd610)
"message-send-and-exit" (0xffffdb18)
"call-interactively" (0xffffde30)
"command-execute" (0xffffe318)


-- 
(domestic pets only, the antidote for overdose, milk.)
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  and http://lars.ingebrigtsen.no/2013/08/twenty-years-of-september.html