(cherry picked from commit 7af19f45)

if nSize was 1 with unicode encoding

nSize would wrap around in lcl_getMaxSafeStrLen

(cherry picked from commit f6368c29)

rework this so we don't read the string, backup and re-read the string again

(cherry picked from commit 8523e57e)

Change-Id: I99f3d4a2ec760228f485d01fce856deb9c068431
Reviewed-on: https://gerrit.libreoffice.org/18690Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>