fix return value of wcs{,n}cmp for extreme wchar_t values

As a result of using simple subtraction to implement the return values
for wcscmp and wcsncmp, integer overflow can occur (producing
undefined behavior, and in practice, a wrong comparison result). This
does not occur for meaningful character values (21-bit range) but the
functions are specified to work on arbitrary wchar_t arrays.

This patch replaces the subtraction with a little bit of code that
orders the characters correctly, returning -1 if the character from
the first string is smaller than the one from the second, 0 if they
are equal and 1 if the character from the first string is larger than
the one from the second.

src/string/wcscmp.c

@@ -3,5 +3,5 @@
 int wcscmp(const wchar_t *l, const wchar_t *r)
 {
 	for (; *l==*r && *l && *r; l++, r++);
-	return *l - *r;
+	return *l < *r ? -1 : *l > *r;
 }

src/string/wcsncmp.c

@@ -3,5 +3,5 @@
 int wcsncmp(const wchar_t *l, const wchar_t *r, size_t n)
 {
 	for (; n && *l==*r && *l && *r; n--, l++, r++);
-	return n ? *l - *r : 0;
+	return n ? (*l < *r ? -1 : *l > *r) : 0;
 }