fix a length 0 bug in bell numbers
fix a length 0 bug in bell numbers

wbhart/flint2,dsroche/flint2,jpflori/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2,dsroche/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,dsroche/flint2,dsroche/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2