[phobos] Clean up patch for std.utf

[Shin Fujishiro]

"Masahiro Nakagawa" <repeatedly at gmail.com> wrote:
> * char version of stride
> 
> I removed assert because the comment says "0xFF meaning s[i] is not the  
> start of of UTF-8 sequence.".
> Until now, my library checked 0xFF :(

Shouldn't it throw an exception?  Consider this use of stride() with a
broken UTF-8 string:

  broken[broken.stride(0) .. $]

It silently succeeds if 'broken' is longer or equal to 255 bytes.


Shin