Fix a memory leak on an error path

A leak of an SSL_SESSION object can occur when decoding a psk extension on
an error path when using TLSv1.3
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2843)

ssl/statem/extensions_srvr.c

@@ -796,6 +796,7 @@ int tls_parse_ctos_psk(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
     s->session = sess;
     return 1;
 err:
+    SSL_SESSION_free(sess);
     return 0;
 }