This reverts commit e8077d01.

* use prev pointer when array do adding

Adds a const qualifier to the strings in the array to avoid discarding
it from arguments.

Signed-off-by: NSanjeev BA <iamsanjeev@gmail.com>

Also first tests for cJSON_Minify.
Thanks @bigric3 for reporting

Thanks  @raiden00pl for reporting

Thanks @AlfieDeng for reporting

If the `string` property of the item that is added is an alias to the
`string` parameter of `add_item_to_object`, and `constant` is false,
`cJSON_strdup` would access the string after it has been freed.

Thanks @hhallen for reporting this in #248.

Thanks @liuyunbin for reporting this in #230

Thanks @liuyunbin for reporting this in #230

Thanks @liuyunbin for reporting this in #230