Fix issue #406

When a peer reconnects, the router socket receives an identity
message containing this peer id. When this happens, the current
implementation crashes.

This patch makes a router socket to silently ignore all identity
messages coming from reconnected peers.

src/msg.hpp

@@ -69,6 +69,7 @@ namespace zmq
         unsigned char flags ();
         void set_flags (unsigned char flags_);
         void reset_flags (unsigned char flags_);
+        bool is_identity () const;
         bool is_delimiter ();
         bool is_vsm ();