Resolves: fdo#50451 Mediawiki formula export + vec use

Unicode Character 'COMBINING RIGHT ARROW ABOVE' wasn't taken into account

Change-Id: I4b5fc5fcfa4efe40fc92935e21af829edadf1dea
Reviewed-on: https://gerrit.libreoffice.org/8093
Reviewed-by: Frédéric Wang <fred.wang@free.fr>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>