Fix err assignment typo

Fixes #36
diff --git a/lib/rac/reader.go b/lib/rac/reader.go
index 4e9710e..8b2c89b 100644
--- a/lib/rac/reader.go
+++ b/lib/rac/reader.go
@@ -438,7 +438,7 @@
 func (r *Reader) seek(offset int64, whence int, limit int64) (int64, error) {
 	if r.concReader.ready() {
 		n, err := r.concReader.seek(offset, whence, limit)
-		r.err = r.err
+		r.err = err
 		return n, err
 	}