check size before looking into the string
check size before looking into the string

Found by Asan. e.g moz233272-2.xpm

Change-Id: Ic563db41dbd4ce7250492e99f3e48a203cfdcf00
Reviewed-on: https://gerrit.libreoffice.org/13686
Reviewed-by: Caolán McNamara <301f5784f9f6aac04077f24f1e5abc31af69d401@redhat.com>
Tested-by: Caolán McNamara <301f5784f9f6aac04077f24f1e5abc31af69d401@redhat.com>

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core