Unsoundness in Pleco crates

[lwz23]

In pleco_engine, the insert_score_depth and insert_score functions use unsafe operations to access an index and modify fields of RootMove. We have noticed that if an invalid index is passed, it can lead to Undefined Behavior (UB). Specifically, the function get_unchecked_mut does not check the validity of the index, so passing an invalid index may cause memory corruption or crashes.
see pleco-rs/Pleco#163 for detail.