bas64 iterator improvement

[Robert Ramey]

 Hello,
> 
> Code that pads base64 output with '=' looks in
> basic_text_oprimitive.ipp like following:
> 
>    std::size_t padding = 2 - count % 3;
>    if(padding > 1)
>        *oi = '=';
>        if(padding > 2)
>            *oi = '=';
> 
> It will give the following transformations of a few simple strings:
> 
> "012" -> "MDEy="
> "0123" -> "MDEyMw"
> "01234" -> "MDEyMzQ"
> "012345" -> "MDEyMzQ1="
> 
> Which violates the principle of the base 64 that the encoded output
> length is multiple of 4 bytes. Right output for the samples above
> should be:
> 
> "012" -> "MDEy"
> "0123" -> "MDEyMw=="
> "01234" -> "MDEyMzQ="
> "012345" -> "MDEyMzQ1"
> 
> 
> The problem with the code above is that the condition (padding > 2)
> is never satisfied. I think that the proper code should look like
> 
>        std::size_t padding = 2 - count % 3;
>        if (padding < 2) {
>          *oi = '=';
>          if (padding > 0)
>            *oi = '=';
>        }

[Robert Ramey]

change checked into trunk