Inkscape, Seamonkey, Chrome and Batik show markers on zero-
length lines, LibreOffice should do that too. The patch
removes the restriction start<>end.

Change-Id: I984baf755df7b2689e00d2a23b40e3f059c0add0
Reviewed-on: https://gerrit.libreoffice.org/22740Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>