SSL: disabled shutdown when there are buffered data.
SSL: disabled shutdown when there are buffered data.

This fixes "SSL_shutdown() failed (SSL: ... bad write retry)" errors
as observed on the second SSL_shutdown() call after SSL shutdown fixes in
09fb2135a589 (1.19.2), notably when HTTP/2 connections are closed due
to read timeouts while there are incomplete writes.

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx