Fix resumption after HRR

Commit 6b1bb98f moved the processing of ClientHello extensions into the
state machine post-processing stage. After processing s->init_num is reset
to 0, so by post-processing we cannot rely on its value. Unfortunately we
were using it to handle the PSK extension. This causes the handshake to
fail.

We were using init_num to figure out the length of ClientHello2 so we can
remove it from the handshake_buffer. The handshake_buffer holds the
transcript of all the messages sent so far. For PSK processing though we
only want to add in a partial ClientHello2. This commit changes things so
we just work out where ClientHello2 starts, working forward from the
beginning of handshake_buffer.

Fixes #2983
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2996)

ssl/statem/extensions.c

@@ -1242,11 +1242,18 @@ int tls_psk_do_binder(SSL *s, const EVP_MD *md, const unsigned char *msgstart,
          * ClientHello - which we don't want - so we need to take that bit off.
          */
         if (s->server) {
-            if (hdatalen < s->init_num + SSL3_HM_HEADER_LENGTH) {
+            PACKET hashprefix, msg;
+
+            /* Find how many bytes are left after the first two messages */
+            if (!PACKET_buf_init(&hashprefix, hdata, hdatalen)
+                    || !PACKET_forward(&hashprefix, 1)
+                    || !PACKET_get_length_prefixed_3(&hashprefix, &msg)
+                    || !PACKET_forward(&hashprefix, 1)
+                    || !PACKET_get_length_prefixed_3(&hashprefix, &msg)) {
                 SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
                 goto err;
             }
-            hdatalen -= s->init_num + SSL3_HM_HEADER_LENGTH;
+            hdatalen -= PACKET_remaining(&hashprefix);
         }
 
         if (EVP_DigestUpdate(mctx, hdata, hdatalen) <= 0) {