Bug report on iswalpha

[Alon Zakai <alonzakai@gmail.com>]

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I think we have encountered a bug in iswalpha, as shown by the following
program:

====
#include <locale.h>
#include <stdio.h>
#include <wctype.h>

int
main(const int argc, const char * const * const argv)
{
  const char * const locale = (argc > 1 ? argv[1] : "C");
  const char * const actual = setlocale(LC_ALL, locale);
  if(actual == NULL) {
    printf("%s locale not supported; skipped locale-dependent code\n",
           locale);
    return 0;
  }
  printf("locale set to %s: %s\n", locale, actual);

  const int result = iswalpha(0xf4); // ô
  printf("iswalpha(\"\xc3\xb4\") = %d\n", result);
  return 0;
}
====

It returns 1 in the final printf, saying that that char is an walpha char,
when I believe it is not. For comparison, glibc reports 0.

Tested on musl 1.0.3 (used in emscripten) and musl trunk on git, same
result.

- Alon

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