Fix a memory leak in tls_parse_stoc_key_share

Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16956)

ssl/statem/extensions_clnt.c

@@ -1830,6 +1830,7 @@ int tls_parse_stoc_key_share(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
         skey = EVP_PKEY_new();
         if (skey == NULL || EVP_PKEY_copy_parameters(skey, ckey) <= 0) {
             SSLfatal(s, SSL_AD_INTERNAL_ERROR, SSL_R_COPY_PARAMETERS_FAILED);
+            EVP_PKEY_free(skey);
             return 0;
         }