cJSON-正确使用防止内存泄漏

主要涉及两个释放内存的函数：cJSON_free和cJSON_Delete，该项目在github上有如下说明：

Printing JSON

Given a tree of cJSON items, you can print them as a string using cJSON_Print.

char *string = cJSON_Print(json);

It will allocate a string and print a JSON representation of the tree into it. Once it returns, you are fully responsible for deallocating it after use with your allocator. (usually free, depends on what has been set with cJSON_InitHooks).

也就是说调用cJSON_Print（以及cJSON_PrintUnformatted）后，必须自行释放内存，对于1.5版本及以上的cJSON可以使用cJSON_free函数，其他版本直接使用free也可，示例代码如下：

char *json_string = cJSON_Print(item);
if (json_string) 
{
    printf("%s\n", json_string);
    cJSON_free(json_string);
}

另一段说明如下：

For every value type there is a cJSON_Create... function that can be used to