Fix an uninitialized memory access in json_pointer.

Add comments describing when the fields of the internal struct json_pointer_get_result are valid.

json_patch.c

@@ -49,9 +49,9 @@ static int json_patch_apply_test(struct json_object **res,
 static int __json_patch_apply_remove(struct json_pointer_get_result *jpres)
 {
 	if (json_object_is_type(jpres->parent, json_type_array)) {
-		return json_object_array_del_idx(jpres->parent, jpres->id.index, 1);
-	} else if (jpres->parent && jpres->id.key) {
-		json_object_object_del(jpres->parent, jpres->id.key);
+		return json_object_array_del_idx(jpres->parent, jpres->index_in_parent, 1);
+	} else if (jpres->parent && jpres->key_in_parent) {
+		json_object_object_del(jpres->parent, jpres->key_in_parent);
 		return 0;
 	} else {
 		return json_object_put(jpres->obj);

json_pointer.c

@@ -190,9 +190,9 @@ static int json_pointer_result_get_recursive(struct json_object *obj, char *path
 		res->parent = parent_obj;
 		res->obj = obj;
 		if (json_object_is_type(res->parent, json_type_array))
-			res->id.index = idx;
+			res->index_in_parent = idx;
 		else
-			res->id.key = path;
+			res->key_in_parent = path;
 	}
 
 	return 0;
@@ -228,11 +228,10 @@ int json_pointer_get_internal(struct json_object *obj, const char *path,
 
 	if (path[0] == '\0')
 	{
-		if (res) {
-			res->parent = NULL;
-			res->obj = obj;
-		}
-		res->id.key = NULL;
+		res->parent = NULL;
+		res->obj = obj;
+		res->key_in_parent = NULL;
+		res->index_in_parent = -1;
 		return 0;
 	}
 
@@ -244,8 +243,8 @@ int json_pointer_get_internal(struct json_object *obj, const char *path,
 	}
 	rc = json_pointer_result_get_recursive(obj, path_copy, res);
 	/* re-map the path string to the const-path string */
-	if (rc == 0 && res->id.key && !json_object_is_type(res->parent, json_type_array))
-		res->id.key = path + (res->id.key - path_copy);
+	if (rc == 0 && json_object_is_type(res->parent, json_type_object) && res->key_in_parent)
+		res->key_in_parent = path + (res->key_in_parent - path_copy);
 	free(path_copy);
 
 	return rc;

json_pointer_private.h

@@ -19,10 +19,11 @@ extern "C" {
 struct json_pointer_get_result {
 	struct json_object *parent;
 	struct json_object *obj;
-	union {
-		const char *key;
-		uint32_t index;
-	} id;
+	// The key of the found object; only valid when parent is json_type_object
+	// Caution: re-uses tail end of the `path` argument to json_pointer_get_internal
+	const char *key_in_parent;
+	// the index of the found object; only valid when parent is json_type_array
+	uint32_t index_in_parent;
 };
 
 int json_pointer_get_internal(struct json_object *obj, const char *path,