Select direct tbody and tr to prevent inner table cause error

[GoogleCodeExporter]

If someone use a custom render to embed another table as the content to a cell, 
this code inside _fnDrawCallBackWithGrouping will cause error:

var nTrs = $('tbody tr', oTable);

One solution is to use direct selector:

var nTrs = $('> tbody > tr', oTable);

Original issue reported on code.google.com by TyraelTong@gmail.com on 2 May 2013 at 5:39