SSL: fixed possible segfault on renegotiation (ticket #845). Skip SSL_CTX_set_tlsext_servername_callback in case of renegotiation. Do nothing in SNI callback as in this case it will be supplied with request in c->data which isn't expected and doesn't work this way. This was broken by b40af2fd1c16 (1.9.6) with OpenSSL master branch and LibreSSL.