Return SSL_ERROR_WANT_READ if SSL_shutdown() encounters handshake data

In the case where we are shutdown for writing and awaiting a close_notify
back from a subsequent SSL_shutdown() call we skip over handshake data
that is received. This should not be treated as an error - instead it
should be signalled with SSL_ERROR_WANT_READ.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6340)