Follow-up to commit 27384cdf7d:
avoid SwIndexReg::Update() correcting the index of CrossRefBookmark away
from 0.
Not sure if it wouldn't be better to remove the requirement that
CrossRefBookmark must have index 0. Or perhaps remove the SwIndex from
their position completely, like AT_PARA frame anchors. But since the
whole bookmark stuff stupidly uses inheritance the base class has
established a requirement that a mark has a position. But then again
the CrossRefBookmark already cannot have a second position, in contrast
to all other marks.
Change-Id: If42cc48724de98151b02469850805f5c77e84c6b
e679b83c38