[3.12] gh-105375: Explicitly initialise all {Pickler,Unpickler}Object fields (GH-105686) (#105710)

All fields must be explicitly initialised to prevent manipulation of
uninitialised fields in dealloc.

Align initialisation order with the layout of the object structs.

(cherry picked from commit ca3cc4b95d66f7527ebe0ba4cdb1907082d9bfc8)

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>