Revert modifications to fmpz_poly_set_length/truncate for sparse polynomial issue.
Revert modifications to fmpz_poly_set_length/truncate for sparse polynomial
issue.

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