It was caused by an unsigned integer underflow i.e. 0 - 1 on size_t.

Change-Id: I579aefa8ffc9e320fadf7180f51711e535fdb778
Reviewed-on: https://gerrit.libreoffice.org/63057
Tested-by: Jenkins
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>