BUG/MINOR: peers: Wrong stick-table update message building.

When creating this patch "CLEANUP: peers: Replace hard-coded values by macros",
we realized there was a remaining place in peer_prepare_updatemsg() where the maximum
of an encoded length harcoded value could be replaced by PEER_MSG_ENCODED_LENGTH_MAXLEN
macro. But in this case, the 1 harcoded value for the header length is wrong. Should
be 2 or PEER_MSG_HEADER_LEN. So, there is a missing byte to encode the length of
remaining data after the header.

Note that the bug was never encountered because even with a missing byte, we could
encode a maximum length which would be (1<<25) (32MB) according to the following
extract of the peers protocol documentation which were from far a never reached limit
I guess:

  I) Encoded Integer and Bitfield.

         0  <= X < 240        : 1 byte  (7.875 bits)  [ XXXX XXXX ]
        240 <= X < 2288       : 2 bytes (11 bits)     [ 1111 XXXX ] [ 0XXX XXXX ]
       2288 <= X < 264432     : 3 bytes (18 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]   [ 0XXX XXXX ]
     264432 <= X < 33818864   : 4 bytes (25 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]*2 [ 0XXX XXXX ]
   33818864 <= X < 4328786160 : 5 bytes (32 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]*3 [ 0XXX XXXX ]
diff --git a/src/peers.c b/src/peers.c
index 9733e1b..0795204 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -406,7 +406,7 @@
 	use_identifier = p->updt.use_identifier;
 	use_timed = p->updt.use_timed;
 
-	cursor = datamsg = msg + 1 + 5;
+	cursor = datamsg = msg + PEER_MSG_HEADER_LEN + PEER_MSG_ENC_LENGTH_MAXLEN;
 
 	/* construct message */