SwDoc::DelNumRule: fix out-of-bounds index access

When nPos == USHRT_MAX, even if later we checked for that case, just for
the sake of an assert(), we already called operator[] with nPos.

(To reproduce: right click on List 1 -> New -> Cancel, results in a
crash.)

Change-Id: I465b5519000b79bd8b2eaa3b3c1ceb80a182a25a