rbindlist segfault fix for factor columns + empty data.tables. Closes #5355.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1123 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Commit 1123 also fixed #5120. Added test.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1124 edb9625f-4e0d-4859-8d74-9fd3b1da38cb