AUTH_FIELD_END looks like it is made for this

avoids mucking with casts and allows more accurate typing

Change-Id: I968a2f7fcfa5534947102918125b095ca3b4a66c
Reviewed-on: https://gerrit.libreoffice.org/11313
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>