BUG/MINOR: ssl: double-free on error path w/ ssl-f-use parser

BUG/MINOR: ssl: double-free on error path w/ ssl-f-use parser

In post_section_frontend_crt_init(), the crt_entry is populated by the
ssl_conf fromt the cfg_crt_node. On error path, the crt_list is
completely freed, including the ssl_conf structure. But the ssl_conf
structure was already freed when freeing the cfg_crt_node.

Fix the issue by doing a crtlist_dup_ssl_conf(n->ssl_conf) in the
crtlist_entry instead of an assignation.

Fix issue #3268.

Need to be backported as far as 3.2. The previous patch which adds the
crtlist_dup_ssl_conf() declaration is needed.