Only copy string for SAL_WARN call, to not print unneeded information

Change-Id: I962ac8a7ed9bc8ee08d1ea31f4f13a0670cbdf3a
Reviewed-on: https://gerrit.libreoffice.org/69232
Tested-by: Jenkins
Reviewed-by: Matteo Casalin <matteo.casalin@yahoo.com>