fix: merge delete_callback omitted by mistake

[zhangsoledad]

#457 make a change

The merge result Vec is turned into a boxed slice and turned into raw pointer by Box::into_raw. In delete_callback, Box::from_raw is called to wrap the raw pointer back and the original memory will be released here.

#481 omit delete_callback by mistake.

According

if (delete_value_ != nullptr) {
      (*delete_value_)(state_, tmp_new_value, new_value_len);
} else {
      free(tmp_new_value);
}

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.into_raw
https://doc.rust-lang.org/nomicon/ffi.html#destructors
I think this is UB.

fix #494

[stanislav-tkach]

👍