New abstractions to access ssl->hostname:
mbedtls_ssl_has_set_hostname_been_called(), mbedtls_ssl_free_hostname().
Use these abstractions to access the hostname with the opportunity for
extra checks in mbedtls_ssl_verify_certificate().
No behavior change except for a new log message.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>