BUG/MAJOR: counters: check for null-deref when looking up an alternate table

Constructions such as sc0_get_gpc0(foo) allow to look up the same key as
the current key but in an alternate table. A check was missing to ensure
we already have a key, resulting in a crash if this lookup is performed
before the associated track-sc rule.

This bug was reported on the mailing list by Neil@iamafreeman and
narrowed down further by Lukas Tribus and Thierry Fournier.

This bug was introduced in 1.5-dev20 by commit "0f791d4 MEDIUM: counters:
support looking up a key in an alternate table".
MINOR: ssl: add ssl_fc_unique_id to fetch TLS Unique ID

The TLS unique id, or unique channel binding, is a byte string that can be
pulled from a TLS connection and it is unique to that connection. It is
defined in RFC 5929 section 3.  The value is used by various upper layer
protocols as part of an extra layer of security.  For example XMPP
(RFC 6120) and EST (RFC 7030).

Add the ssl_fc_unique_id keyword and corresponding sample fetch method.
Value is retrieved from OpenSSL and base64 encoded as described in RFC
5929 section 3.
BUILD: ssl: previous patch failed

This is a minor error, s/SMP_T_CSTR/SMP_T_STR + SMP_F_CONST.
Merge branch 'master' of http://git.1wt.eu/git/haproxy