codec: Fix demux returning data to pool after Unmarshal

The passed slice cannot be guaranteed to be from the pool, therefore it
should not be returned there during Release.

codec.go

@@ -162,6 +162,11 @@ func Unmarshal(data []byte) (*Message, error) {
 	}
 
 	arena := MultiSegment(nil)
+
+	// We cannot return the data parameter to the bufferpool because we
+	// don't know where it came from.
+	arena.bp = nil
+
 	if err := arena.demux(hdr, data); err != nil {
 		return nil, exc.WrapError("unmarshal", err)
 	}