Don't generate an unnecessary Diffie-Hellman key in TLS 1.3 clients.

tls_parse_stoc_key_share was generating a new EVP_PKEY public/private
keypair and then overrides it with the server public key, so the
generation was a waste anyway. Instead, it should create a
parameters-only EVP_PKEY.

(This is a consequence of OpenSSL using the same type for empty key,
empty key with key type, empty key with key type + parameters, public
key, and private key. As a result, it's easy to mistakenly mix such
things up, as happened here.)
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/9445)

ssl/statem/extensions_clnt.c

@@ -1900,8 +1900,8 @@ int tls_parse_stoc_key_share(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
         return 0;
     }
 
-    skey = ssl_generate_pkey(ckey);
-    if (skey == NULL) {
+    skey = EVP_PKEY_new();
+    if (skey == NULL || EVP_PKEY_copy_parameters(skey, ckey) <= 0) {
         SSLfatal(s, SSL_AD_INTERNAL_ERROR, SSL_F_TLS_PARSE_STOC_KEY_SHARE,
                  ERR_R_MALLOC_FAILURE);
         return 0;