Ensure we are in accept state in DTLSv1_listen

Calling SSL_set_accept_state() after DTLSv1_listen() clears the state, so
SSL_accept() no longer works. In 1.0.2 calling DTLSv1_listen() would set
the accept state automatically. We should still do that.

Fixes #1989
Reviewed-by: NAndy Polyakov <appro@openssl.org>

ssl/d1_lib.c

@@ -432,6 +432,11 @@ int DTLSv1_listen(SSL *s, BIO_ADDR *client)
     BIO_ADDR *tmpclient = NULL;
     PACKET pkt, msgpkt, msgpayload, session, cookiepkt;
 
+    if (s->handshake_func == NULL) {
+        /* Not properly initialized yet */
+        SSL_set_accept_state(s);
+    }
+
     /* Ensure there is no state left over from a previous invocation */
     if (!SSL_clear(s))
         return -1;