BUG/MINOR: ssl: fix smp_fetch_ssl_fc_session_id

BUG/MINOR: ssl: fix smp_fetch_ssl_fc_session_id

Dmitry Sivachenko reported the following build warning using Clang
which is a real bug :

src/ssl_sock.c:4104:44: warning: address of 'smp->data.str.len' will always
      evaluate to 'true' [-Wpointer-bool-conversion]
        if (!smp->data.str.str || !&smp->data.str.len)

The impact is very low however, it will return an empty session_id
instead of no session id when none is found.

The fix should be backported to 1.5.