look for OUString being appended to in a loop, better to use
OUStringBuffer to accumulate the results.

Change-Id: Ia36e06e2781a7c546ce9cbad62727aa4c5f10c4b
Reviewed-on: https://gerrit.libreoffice.org/58092
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>