base64 not work well on vc2019

[a645162]

Expression: cannot seek string iterator after end

The program is normal under the clang compiler, but on vc2019, Microsoft will report an error on the judgment condition of your "for" statement. If the judgment of the maximum pointer of the iterator exceeds the error, the debug channel will report an error, and the output of the Release channel will be abnormal.
Assuming the string length is "length", then it is normal when you execute begin+length<end, but after each round of the loop, the pointer is shifted back by 3 bits, then begin+length+1<end and begin+length+2<end two judgment statements will report an error.

I have a solution:
Change
for (auto it = begin; it < end; it += 3) {
to

auto length = end - begin;
for (auto i = 0; i < length; i += 3) {
				auto it = begin + i;

Change

if (it + 1 < end) {
if (it + 2 < end) {

to

if (i + 1 < length) {
if (i + 2 < length) {

Personally think this is the easiest solution.