[tls] Guard against resuming from an empty resumption master secret

There should be no circumstance that leads to a session being resumed
without having a valid session resumption secret that was stored by a
successfully established previous connection.

As an additional layer of defence in depth, clear the "keyed" and
"bound" flags in the key schedule if it is ever resumed from an empty
session resumption secret.

Signed-off-by: Michael Brown <mcb30@ipxe.org>