SSL: moved c->ssl->handshaked check in server name callback.

SSL: moved c->ssl->handshaked check in server name callback.

Server name callback is always called by OpenSSL, even
if server_name extension is not present in ClientHello.  As such,
checking c->ssl->handshaked before the SSL_get_servername() result
should help to more effectively prevent renegotiation in
OpenSSL 1.1.0 - 1.1.0g, where neither SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS
nor SSL_OP_NO_RENEGOTIATION is available.