Fix displaying sort icon in the first column in case of unsorted grid

If no additional column are added (by `rownumbers: true` for example) and no `sortname` are defined (so the default `sortname: ""` and `lastsort: 0` are used) then the sorting icon will be wrong displayed in the first column.

[The demo](http://www.ok-soft-gmbh.com/jqGrid/gridWithoutSort0.htm) can be used to reproduce the problem. [Another demo](http://www.ok-soft-gmbh.com/jqGrid/gridWithoutSort1.htm) uses the suggested fix and it seems work correctly.