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libc: Fix signess of pointer in mbtowc()

We have to access the elements of the char array using
an unsigned pointer, because otherwise we can have
sign extension that can modify the behaviour of the
function.

--- a/src/libc/stdlib/mbtowc.c

+++ b/src/libc/stdlib/mbtowc.c

@@ -5,7 +5,7 @@

 int

 mbtowc(wchar_t *restrict pwc, const char *restrict s, size_t n)

 {

-	const char *t = s;

+	unsigned char *t = (unsigned char *) s;

 	unsigned long wc;

 	unsigned c;

 	size_t len;

@@ -30,5 +30,7 @@

 return_code:

 	if (pwc)

 		*pwc = wc;

-	return (*s) ? t - s : 0;

+	if (*s == '\0')

+		return 0;

+	return t - (unsigned char *) s;

 }