BUG/MEDIUM: ssl: fix the id length check within smp_fetch_ssl_fc_session_id()

BUG/MEDIUM: ssl: fix the id length check within smp_fetch_ssl_fc_session_id()

After we call SSL_SESSION_get_id(), the length of the id in bytes is
stored in "len", which was never checked. This could cause unexpected
behavior when using the "ssl_fc_session_id" or "ssl_bc_session_id"
fetchers (eg. the result can be an empty value).

The issue was introduced with commit 105599c ("BUG/MEDIUM: ssl: fix
several bad pointer aliases in a few sample fetch functions").

This patch must be backported to 2.1, 2.0, and 1.9.