fix failure to distinguish input/match failure in wide %[ scanf

this also includes a related fix for vswscanf's read function, which
was returning a spurious (uninitialized) character for empty strings.

src/stdio/vfwscanf.c

@@ -264,7 +264,9 @@ int vfwscanf(FILE *f, const wchar_t *fmt, va_list ap)
 			}
 			if (width) ungetwc(c, f);
 
-			if (!gotmatch) goto match_fail;
+			if (!gotmatch)
+				if (c>=0) goto match_fail;
+				else goto input_fail;
 
 			if (*p==']') p++;
 			while (*p!=']') {

src/stdio/vswscanf.c

@@ -17,7 +17,7 @@ static size_t wstring_read(FILE *f, unsigned char *buf, size_t len)
 	f->rend = f->buf + k;
 	f->cookie = (void *)src;
 
-	if (!len) return 0;
+	if (!len || !k) return 0;
 
 	*buf = *f->rpos++;
 	return 1;