[tls] Guard against a premature server Finished

A malicious server that immediately sends a Finished record (without
ever having sent a ServerHello) will currently cause tls_prf() to get
stuck in an infinite loop attempting to generate pseudorandom data
using the null digest algorithm.

Fix by checking that the key schedule digest size is non-zero
(i.e. that the digest is not the null digest) before attempting to
process the Finished record.

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