BUILD: ssl: fix null-deref warning in ssl_fc_cipherlist_str sample fetch

BUILD: ssl: fix null-deref warning in ssl_fc_cipherlist_str sample fetch

Gcc 6.4 detects a potential null-deref warning in smp_fetch_ssl_fc_cl_str().
This one is not real since already addressed a few lines above. Let's use
__objt_conn() instead of objt_conn() to avoid the extra test that confuses
it.

This could be backported to 1.8.