tls: fix refresh_certs crash when disabled
tls: fix refresh_certs crash when disabled

segfaults server if refresh_certs command is run with TLS compiled in
but not enabled.

memcached/memcached,dormando/memcached,dormando/memcached,memcached/memcached,dormando/memcached,dormando/memcached,dormando/memcached,memcached/memcached,memcached/memcached,memcached/memcached