Minor cleanup of scalar_addmul_fmpz.
Minor cleanup of scalar_addmul_fmpz.

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