fdo#85872: sw: fix crash in insert index dialog

Looks like the old code would create a link starting at the beginning in
this case, so let's do the same.

(regression from 94b296d5416dd71d721ad16216b50bce79e3dc04)

Change-Id: Idcd17ae51c478aa5c2a000c7b33a8244f06bd166