Return correct next state from Read

gigaparsec.go

@@ -127,8 +127,8 @@ type State[In any] struct {
 
 func (s State[In]) Read(dst []In) (n uint64, next State[In], err error) {
 	nread, err := s.r.ReadAt(dst, int64(s.pos))
-	if n > 0 {
-		s.pos += uint64(n)
+	if nread > 0 {
+		s.pos += uint64(nread)
 	}
 	return uint64(nread), s, err
 }