Switch finish_handshake to release_current_message.

With the previous DTLS change, the dispatch layer only cares about the
end of the handshake to know when to drop the current message. TLS 1.3
post-handshake messages will need a similar hook, so convert it to this
lower-level one.

BUG=83

Change-Id: I4c8c3ba55ba793afa065bf261a7bccac8816c348
Reviewed-on: https://boringssl-review.googlesource.com/8989
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/ssl/d1_pkt.c b/ssl/d1_pkt.c
index ceb6e17..ab9b911 100644
--- a/ssl/d1_pkt.c
+++ b/ssl/d1_pkt.c
@@ -224,7 +224,7 @@
     }
 
     if (msg_hdr.type == SSL3_MT_FINISHED &&
-        msg_hdr.seq == ssl->d1->handshake_read_seq) {
+        msg_hdr.seq == ssl->d1->handshake_read_seq - 1) {
       if (msg_hdr.frag_off == 0) {
         /* Retransmit our last flight of messages. If the peer sends the second
          * Finished, they may not have received ours. Only do this for the