slightly simplify _fmpz_poly_mul
slightly simplify _fmpz_poly_mul

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