Fix calling of jqGridCellSelect or editCell with iCol=-1

In case of selection multiple cells of the same row with respect of mouse the click event will be fired with DOM of `<tr>` as the e.target. Suggested fix solves the problem. For more details see [the post](http://www.trirand.com/blog/?page_id=393/bugs/oncellselect-and-editcell-can-be-called-with-ci-1).