Pickle `load_build` function checks if `slotstate` is False, not a dict

Bug report

Bug description:

This issue is similar to Issue https://github.com/python/cpython/issues/128965. Inside of the load_build() function, Python pickle assumes that slotstate is a dictionary while C _pickle explicitly checks to ensure it's a dictionary. The behavior of these two implementations diverge when slotstate is falsey but not a dictionary.

payload:      b'NN\x8f\x86b.'

pickle:       None
_pickle.c:    FAILURE slot state is not a dictionary
pickletools:
    0: N    NONE
    1: N    NONE
    2: \x8f EMPTY_SET
    3: \x86 TUPLE2
    4: b    BUILD
    5: .    STOP
highest protocol among opcodes = 4

In this case, opcode 2 (EMPTY_SET) is slotstate. When running in pickle.py, this is falsey and thus the code inside setting attributes will not be run, causing the pickle to continue deserializing without any errors.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Lib/pickle.py#L1868

When running in _pickle.c, this value is not a dictionary, causing an error to be thrown.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Modules/_pickle.c#L6903

I guess also in general I'm not understanding what the if slotstate in pickle.py is meant to do anyway. If it's supposed to check if slotstate == None, then the C _pickle module doesn't do that correctly. It checks if slotstate is NULL, which is not the same as PyNone.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Modules/_pickle.c#L6899

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-144582

Compare Lib/pickle.py's load_build implementation with Modules/_pickle.c, using the payload b'NN\x8f\x86b.' to reproduce the divergent behavior. Review linked PR gh-144582 and confirm that both implementations consistently reject a falsey non-dictionary slotstate, with the relevant pickle behavior covered by tests if the PR identifies them.