Set connection's peer id before looking up handler method.

Otherwise, in the case of an unexpected packet, an incorrect message id
would be used.
Also, refuse setting a None peer_id on connection.

neo/lib/connection.py

@@ -464,6 +464,7 @@ class Connection(BaseConnection):
         self.uuid = uuid
 
     def setPeerId(self, peer_id):
+        assert peer_id is not None
         self.peer_id = peer_id
 
     def getPeerId(self):

neo/lib/handler.py

@@ -46,12 +46,12 @@ class EventHandler(object):
     def dispatch(self, conn, packet, kw={}):
         """This is a helper method to handle various packet types."""
         try:
+            conn.setPeerId(packet.getId())
             try:
                 method = getattr(self, packet.handler_method_name)
             except AttributeError:
                 raise UnexpectedPacketError('no handler found')
             args = packet.decode() or ()
-            conn.setPeerId(packet.getId())
             method(conn, *args, **kw)
         except UnexpectedPacketError, e:
             self.__unexpectedPacket(conn, packet, *e.args)