transport/unix: Fixing segfault due to errorneous creation of multiple UnixTransport::Impl instances

The bug showed itself when multiple expressInterest's were issued before
running processEvents.

Change-Id: I7e60d0ae625777e06471aca00707d7b88e68a5c8
diff --git a/include/ndn-cpp/transport/unix-transport.hpp b/include/ndn-cpp/transport/unix-transport.hpp
index b7b65a9..c2ee204 100644
--- a/include/ndn-cpp/transport/unix-transport.hpp
+++ b/include/ndn-cpp/transport/unix-transport.hpp
@@ -34,7 +34,7 @@
   std::string unixSocket_;
 
   class Impl;
-  std::auto_ptr<Impl> impl_;
+  ptr_lib::shared_ptr<Impl> impl_;
 };
 
 }