// all of the header fields in r.
func (r *LineReader) readHeader(compDir string) error {
	buf := &r.buf

	// Read basic header fields [DWARF2 6.2.4].
	hdrOffset := buf.off
	unitLength, dwarf64 := buf.unitLength()
	r.endOffset = buf.off + unitLength
	if r.endOffset > buf.off+Offset(len(buf.data)) {
		return DecodeError{"line", hdrOffset, fmt.Sprintf("line table end %d exceeds section size %d", r.endOffset, buf.off+Offset(len(buf.data)))}