Restore the SSL_set_session_id_context() call to OpenSSL renegotiation.

This reverts the removal of the call in commit (272923a0). It turns out it
wasn't superfluous after all: without it, renegotiation fails if a client
certificate was used. The rest of the changes in that commit are still OK
and not reverted.

Per investigation of bug #12769 by Arne Scheffer, although this doesn't fix
the reported bug yet.

src/backend/libpq/be-secure-openssl.c

@@ -595,6 +595,10 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 		 */
 		SSL_clear_num_renegotiations(port->ssl);
 
+		/* without this, renegotiation fails when a client cert is used */
+		SSL_set_session_id_context(port->ssl, (void *) &SSL_context,
+								   sizeof(SSL_context));
+
 		if (SSL_renegotiate(port->ssl) <= 0)
 			ereport(COMMERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),