Fixed wrong buffer end detection in v2_decoder.

zero-copy msg_t::init cannot be used when the message exceeds either
the buffer end or the last received byte. To detect this, the buffer
is now resized to the numnber of received bytes.

src/v2_decoder.hpp

@@ -80,9 +80,16 @@ namespace zmq
             return buf;
         }
 
+        void resize(size_t new_size)
+        {
+            bufsize = new_size;
+        }
+
     private:
         unsigned char* buf;
         size_t bufsize;
+        size_t maxsize;
+        zmq::atomic_counter_t* msg_refcnt;
     };
 
     //  Decoder for ZMTP/2.x framing protocol. Converts data stream into messages.