bpo-38876: Raise pickle.UnpicklingError when loading an item from memo for invalid input (GH-17335)

The previous code was raising a `KeyError` for both the Python and C implementation.
This was caused by the specified index of an invalid input which did not exist
in the memo structure, where the pickle stores what objects it has seen.
The malformed input would have caused either a `BINGET` or `LONG_BINGET` load
from the memo, leading to a `KeyError` as the determined index was bogus.

https://bugs.python.org/issue38876



https://bugs.python.org/issue38876