Return a fatal error if application data is encountered during shutdown

Currently if you encounter application data while waiting for a
close_notify from the peer, and you have called SSL_shutdown() then
you will get a -1 return (fatal error) and SSL_ERROR_SYSCALL from
SSL_get_error(). This isn't accurate (it should be SSL_ERROR_SSL) and
isn't persistent (you can call SSL_shutdown() again and it might then work).

We change this into a proper fatal error that is persistent.

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