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/raw_decoder.hpp

@@ -56,7 +56,7 @@ namespace zmq
 
         virtual msg_t *msg () { return &in_progress; }
 
-
+        virtual void resize_buffer(size_t) {}
     private: