Implement server side of PSK extension construction

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)

ssl/s3_lib.c

diff --git a/ssl/s3_lib.c b/ssl/s3_lib.c
index 6c74bd1..35684f4 100644
--- a/ssl/s3_lib.c
+++ b/ssl/s3_lib.c
@@ -4113,6 +4113,9 @@
                 rv = tls13_generate_secret(s, ssl_handshake_md(s), NULL, NULL,
                                            0,
                                            (unsigned char *)&s->early_secret);
+            else
+                rv = 1;
+
             rv = rv && tls13_generate_handshake_secret(s, pms, pmslen);
         } else {
             /* Generate master secret and discard premaster */