Resolves: fdo#84809 crash when button is too short for text

regression from 4c539fac018dfd44cd8db52161a8cb930c627da7

commit 4c539fac018dfd44cd8db52161a8cb930c627da7
Date:   Tue Dec 17 05:18:35 2013 -0600
    vcl get rid of xub_StrLen and STRING_LEN in outdev3

before this there was a dubious cast which hid the problem

Change-Id: I7c01c0f0ac211196a62acaf02297f29ae829403d
(cherry picked from commit 3144b8deaf8397d719b7ad7a56b309655a53d90a)
Reviewed-on: https://gerrit.libreoffice.org/11871
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>