Avoid an infinite loop in SSL_shutdown()

If the peer closed the write side of the connection and we have not
yet received the close_notify, SSL_shutdown() makes an extra read to
try and read the peer's close_notify from the pipe. In that situation,
we receive EOF. The legacy stack will return -1 while the TLSv1.3
stack will end up returning 0.

Since the documentation is not super explicit about what should be
done if SSL_shutdown() returns 0, some applications will enter an
infinite loop. The code and documentation indicate that SSL_shutdown()
should only be called once more if it returned 0. Newer versions
of the OpenSSL documentation explicitly say that one should call
SSL_read() if SSL_shutdown() returns 0 in order to retrieve the
close_notify. Doing this would also have avoided this infinite loop.

Reported by Carsten Arzig and bluhm with a test case extracted from the
syslogd tests using IO::Socket::SSL, which has such an infinite loop.

ok bluhm jsing

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