Fixed leak in dogroups when it (rarely) writes less than allocated (non-trivial aggregations) to fully resolve #2648.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1059 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'