Only use the fallback mtu after 2 unsuccessful retransmissions if it is less

than the mtu we are already using
Reviewed-by: NTim Hudson <tjh@openssl.org>

ssl/d1_lib.c

@@ -475,13 +475,17 @@ void dtls1_stop_timer(SSL *s)
 
 int dtls1_check_timeout_num(SSL *s)
 	{
+	unsigned int mtu;
+
 	s->d1->timeout.num_alerts++;
 
 	/* Reduce MTU after 2 unsuccessful retransmissions */
 	if (s->d1->timeout.num_alerts > 2
 			&& !(SSL_get_options(s) & SSL_OP_NO_QUERY_MTU))
 		{
-		s->d1->mtu = BIO_ctrl(SSL_get_wbio(s), BIO_CTRL_DGRAM_GET_FALLBACK_MTU, 0, NULL);		
+		mtu = BIO_ctrl(SSL_get_wbio(s), BIO_CTRL_DGRAM_GET_FALLBACK_MTU, 0, NULL);
+		if(mtu < s->d1->mtu)
+			s->d1->mtu = mtu;
 		}
 
 	if (s->d1->timeout.num_alerts > DTLS1_TMO_ALERT_COUNT)