sw: ww8: fix another ~SwIndexReg() assertion

If the position is the same as the body text anchor position, don't
delete the node.  Probably something should have inserted more nodes
between StartApo() and StopApo().

Change-Id: I41110a47d840e764f6d2a24e43bf6938b1282972
(cherry picked from commit 404f16e97f1c2fcd8f9a1297bdfa46cba970467e)
Reviewed-on: https://gerrit.libreoffice.org/11046
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>