RT2772: accept empty SessionTicket

RFC 5077 section 3.3 says:
If the server determines that it does not want to include a
ticket after it has included the SessionTicket extension in the
ServerHello, then it sends a zero-length ticket in the
NewSessionTicket handshake message.

Previously the client would fail upon attempting to allocate a
zero-length buffer. Now, we have the client ignore the empty ticket and
keep the existing session.

Reviewed-by: Matt Caswell <matt@openssl.org>