Use a special marker as ssl->hostname if mbedtls_ssl_set_hostname() has been
called with NULL. If mbedtls_ssl_set_hostname() has never been called, the
field is NULL, as before.
No behavior change apart from now emitting a different log message depending
on whether mbedtls_ssl_set_hostname() has been called with NULL or not at all.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>