vcl get rid of xub_StrLen and STRING_LEN in outdev3

a new log section (sal.rtl.xub) is used to display alert in case of
suspicious len == 0xFFFF (aka STRING_LEN)

Change-Id: I3ed2aa7896e12592be9e003580dd6c8eda4add5e
Reviewed-on: https://gerrit.libreoffice.org/7117
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>