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[lib/c] Fix memcmp()

Char can be signed and the substraction of signed types can produce
unexpected results if it doesn't fit in an integer.

--- a/lib/c/memcmp.c

+++ b/lib/c/memcmp.c

@@ -4,9 +4,10 @@

 int

 memcmp(const void *s1, const void *s2, size_t n)

 {

-	char *s = (char *) s1, *t = (char *) s2;

+	const unsigned char *s = (unsigned char *) s1;

+	const unsigned char *t = (unsigned char *) s2;

-	while (n > 0 && *s == *t)

-		--n, ++s, ++t;

+	for ( ; n > 0 && *s++ == *t++; --n)

+		;

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

 }