Avoid accessing 1 character after a string

Found while trying to reproduce fdo#83141, but not related to
that - it just happened to trigger the relevant assert on a dbgutil
build.

The change to TextSearch::NSrchFrwrd() fixes the crash triggered
by reproducing the above bug.
The change to TextSearch::NSrchBkwrd() is by analogy but seems an
equally good idea.

Change-Id: I68c2c87b632dd53453f92394519a06f62e41bbad
Reviewed-on: https://gerrit.libreoffice.org/11830
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
(cherry picked from commit 555a7a601b33c18472be7e99b0f9c8acb05a82d7)
Reviewed-on: https://gerrit.libreoffice.org/11846