Small improvement to mullow_karatsuba_n.c
Small improvement to mullow_karatsuba_n.c

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