editeng: avoid autocorrect crash

Happened for me with nFndPos = 30 and nEndPos = 20, and a rTxt that
probably contained "->" twice.

Change-Id: I534c60a5904249e25fdefe45639b4512e6f138c5
Reviewed-on: https://gerrit.libreoffice.org/11344
Reviewed-by: Németh László <nemeth@numbertext.org>
Tested-by: Michael Stahl <mstahl@redhat.com>