Fix #81714: segfault when serializing finalized HashContext

We must not allow to serialize already finalized `HashContext`s, since
the internal context is already freed. Since there is

Fix #81714: segfault when serializing finalized HashContext

We must not allow to serialize already finalized `HashContext`s, since
the internal context is already freed. Since there is not much point
in serializing finalized `HashContext`s, we just bail out in that case.

Closes GH-8265.

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