Fix double-free in OpenSSL's SSL ECDH code.

It has yet to be determined if this warrants a FreeBSD Security
Advisory, but we might as well get it fixed in the normal branches.

Obtained from:	OpenSSL CVS
Security:	CVE-2010-2939
X-MFC after:	Not long...

crypto/openssl/ssl/s3_clnt.c

@@ -1377,6 +1377,7 @@ int ssl3_get_key_exchange(SSL *s)
 		s->session->sess_cert->peer_ecdh_tmp=ecdh;
 		ecdh=NULL;
 		BN_CTX_free(bn_ctx);
+		bn_ctx = NULL;
 		EC_POINT_free(srvr_ecpoint);
 		srvr_ecpoint = NULL;
 		}