and apply it to some methods in OString and OUString

Change-Id: I30e91f961b6d310799d3641f68b7ed54b3080f3a
Reviewed-on: https://gerrit.libreoffice.org/38020Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>