cool#6580 sw: fix infinite loop when changing document language

cool#6580 sw: fix infinite loop when changing document language

If there's a footnote in the document, changing the document langauge
goes into an infinite loop in FindParentText(), because the selection
created by ExtendedSelectAll(true) is actually invalid, apparently
the intention is that only very limited functions may be called while it
is active.

Don't handle this invalid "very" extended selection like one created by
ExtendedSelectAll(false).

(regression from commit d81379db730a163c5ff75d4f3a3cddbd7b5eddda)

Change-Id: Icf1032715cf2e0a05bf485039c483440c08bb6bb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/152797
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
(cherry picked from commit ca9341cf60f3f9350662d30b61f6eadefca24667)