Problem with blob_t initialisation fixed.

HP's version of STL doesn't allow for initialisation of basic_string
(blob_t) using NULL pointer, while SGI's implementation is OK with
that. Fixed.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/object.cpp

@@ -81,8 +81,9 @@ void zmq::object_t::process_command (command_t &cmd_)
 
     case command_t::attach:
         process_attach (cmd_.args.attach.engine,
+            cmd_.args.attach.peer_identity ?
             blob_t (cmd_.args.attach.peer_identity,
-            cmd_.args.attach.peer_identity_size));
+            cmd_.args.attach.peer_identity_size) : blob_t ());
         process_seqnum ();
         break;