xub_StrLen and tools/string.hxx final straw

Thre is still some 0xffff limit left and possibly some
less than gracefully handled overflow/error cases

Change-Id: I00957ee3a30b02f73918ea49d7353056263dc638
Reviewed-on: https://gerrit.libreoffice.org/7787
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>