GeezerVison

Clearing an Unpickler memo left its sequential index and sparse entries intact, so reusing the unpickler for an independent record could raise UnpicklingError or retrieve an object from a previous record. Replace the memo and reset all of its state before releasing the old entries. Backport to 3.15; manually backport only the counter reset to 3.14, which has no sparse memo.
(cherry picked from commit cc36604)

Co-authored-by: Jelle Zijlstra [email protected]

Backport of #160173 to 3.14.

Reset the sequential memo index when clearing the unpickler's memo. Python 3.14 stores all memo entries in an array, so only the counter reset is needed here.

Ran test_pickle on a free-threaded 3.14 build, including clearing and reusing the unpickler for a second record.

• Issue: Fix minor safety issues #159739