BUG/MEDIUM: ssl: avoid double free when releasing bind_confs

BUG/MEDIUM: ssl: avoid double free when releasing bind_confs

ssl_sock functions don't mark pointers as NULL after freeing them. So
if a "bind" line specifies some SSL settings without the "ssl" keyword,
they will get freed at the end of check_config_validity(), then freed
a second time on exit. Simply mark the pointers as NULL to fix this.
This fix needs to be backported to 1.7 and 1.6.