[Libreoffice] [REVIEW] fix for crash in new ww8 field code

[Caolán McNamara]

i.e. accessing top of an empty stack, noticed when doing regression
tests on #i38880#. Proposing cherry-pick to 3-4

http://cgit.freedesktop.org/libreoffice/writer/commit/?id=156c5bd4b04402f8dd724da6c8f984c4d63e4171

C.