guard against null pointer access on LRU function list, fdo#60036

... which may have happened if the config's LRU contained enums of
non-existing functions, i.e. if they were shifted by an update.

Change-Id: I9a005f41cc74c530cf37d1782ce8c53b8f5dce6e
(cherry picked from commit a94f0f92e8b09f6cd3989b646500ff5814274621)
Reviewed-on: https://gerrit.libreoffice.org/7750
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>