fdo#72219: Fix for corruption of symbols in docx

Issue:
OUString uses UTF-16, so for a Unicode surrogate character there are 2
values stored, not just 1.
So we are getting assert failure in "rtl_uString_iterateCodePoints" method.

erAck: Underlying cause was that the dictionary breakiterator misused
UTF-16 positions as Unicode code point positions.

Change-Id: I923485f56c2d879b63687adaea2b489a3479991c
Reviewed-on: https://gerrit.libreoffice.org/6955
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
(cherry picked from commit d8fd15875901d584a4bbcc07c927fa20332e4841)
Reviewed-on: https://gerrit.libreoffice.org/7322