bug#16544: Optimazation for is_mb_middle

[Paul Eggert]

Thanks for the further patch.  Some comments:

> +      if (!mbsinit (&mbs))
> +        memset (&mbs, 0, sizeof mbs);

Why bother with mbsinit?  Just do the memset, or better yet declare and 
initialize mbs here.

> +      mbclen_guess[i] = mbrlen ((const char *) &i, 1, &mbs);

This assumes a little-endian machine, which is not portable.  Please use 
something like this instead:

  for (i = CHAR_MIN; i <= CHAR_MAX; i++)
    {
       char c = i;
       unsigned char uc = i;
mbstate_t mbs = { 0 };
       mbclen_guess[uc] = mbrlen (&c, 1, &mbs);
    }

Here I'm using a style that avoids casts, as casts in general can be 
dangerous.

A minor question about naming: in what sense is mbclen_guess a guess?  
It doesn't seem to be guessing anything. Perhaps rename it to mbclen_cache?