fdo#80663: sw: fix crash on Undo of table row/column delete

The _FndBox appears to handle an empty table lines vector, but evidently
it does not check its indexes properly in this case, which crashes with
STL containers.

(regression from bc0c939270a45469b8f22008288b5e4a4d9c9861)

Change-Id: Id864ea12bc97e61c5251f470e446b6b8f69cb054