utf8: fix iconv error detection

iconv(3) returns "(size_t) -1" on error.  Make sure that we cast the
"-1" properly when checking for this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

utf8.c

@@ -529,7 +529,7 @@ char *reencode_string_iconv(const char *in, size_t insz, iconv_t conv, int *outs
 	while (1) {
 		size_t cnt = iconv(conv, &cp, &insz, &outpos, &outsz);
 
-		if (cnt == -1) {
+		if (cnt == (size_t) -1) {
 			size_t sofar;
 			if (errno != E2BIG) {
 				free(out);