Non-BMP characters use two sal_Unicodes. It is necessary
to advnace with OUString::iterateCodePoints safely instead of
nCharPos + 1.

Change-Id: Iea1005f05421b1021a9b3ff611439357d4dcba9e
Reviewed-on: https://gerrit.libreoffice.org/73326
Tested-by: Jenkins
Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>