Clear memo map after op code STOP

[viirya]

Each unpickling should be independent. The memo map should not be reused without clearing between each loads/load call.

The added test shows the case that can cause wrong unpickled results. The serialized bytes were dumped from Python.

[irmen]

thanks for this! I'll fix the C# code myself.

[viirya]

Thanks!

[irmen]

note that Python's own Unpickler works a bit differently and does reuse the memo over sequential calls, but it reads pickles from a single stream passed at construction time.