It's actually possible for Seek to be invoked with a position that is
neither 0 nor >= the current position:

lcl_CalcOptRepaint(), called from SwTextFormatter::FormatLine(), will
move backwards via some funny heuristic and call GetCharRect(), which
then seeks.

Change-Id: If061a768b9a0155d9a95b190c44e7048312f4ba3
(cherry picked from commit c985eba6)