SunStudio fixed.

With SunStudio's implementation of STL basic_string constructor
doesn't accept NULL as a parameter even though size of string
is set to zero. Fixed.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/object.cpp

@@ -85,8 +85,8 @@ void zmq::object_t::process_command (command_t &cmd_)
 
     case command_t::bind:
         process_bind (cmd_.args.bind.in_pipe, cmd_.args.bind.out_pipe,
-            blob_t (cmd_.args.bind.peer_identity,
-            cmd_.args.bind.peer_identity_size));
+            cmd_.args.bind.peer_identity ? blob_t (cmd_.args.bind.peer_identity,
+            cmd_.args.bind.peer_identity_size) : blob_t ());
         process_seqnum ();
         break;