[REVIEW-3-6 3-6-3] fix for fdo#55734 ,don't access index 0 in an empty vector

[Markus Mohrhard]

Hey,

[1] fixes a crash that happens when we try to access the first element
of an empty vector.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=132740681c7412f4d678ff0768b9807502525f76