Fixed segfault with forder on character strings longer than 20,  thanks to Zach Mayer for reporting from v1.8.11

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1144 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added NEWS item and test for forder cradix segfault when nchar>20.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1145 edb9625f-4e0d-4859-8d74-9fd3b1da38cb