Issue #407: fix incorrect casts in calls to ctype functions (isdigit and isspace) so we don't crash when asserts are enabled on certain platforms and characters > 128 are parsed.

json_pointer.c

@@ -44,7 +44,7 @@ static int is_valid_index(struct json_object *jo, const char *path, int32_t *idx
 	/* this code-path optimizes a bit, for when we reference the 0-9 index range in a JSON array
 	   and because leading zeros not allowed */
 	if (len == 1) {
-		if (isdigit((int)path[0])) {
+		if (isdigit((unsigned char)path[0])) {
 			*idx = (path[0] - '0');
 			goto check_oob;
 		}
@@ -58,7 +58,7 @@ static int is_valid_index(struct json_object *jo, const char *path, int32_t *idx
 	}
 	/* RFC states base-10 decimals */
 	for (i = 0; i < len; i++) {
-		if (!isdigit((int)path[i])) {
+		if (!isdigit((unsigned char)path[i])) {
 			errno = EINVAL;
 			return 0;
 		}