gh-71810: Fix _PyLong_AsByteArray() undefined behavior

[vstinner]

Don't read p[-1] when p is an empty string: when n==0.

• Issue: int.to_bytes() and int.from_bytes(): raise ValueError when bytes count is zero #71810

[vstinner]

cc @skirpichev @serhiy-storchaka

[serhiy-storchaka]

LGTM. 👍

[miss-islington-app]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-138883 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-138884 is a backport of this pull request to the 3.13 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot ARM64 MacOS M1 Refleaks NoGIL 3.13 (tier-2) has failed when building commit a902058.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1396/builds/1590) and take a look at the build logs.
4. Check if the failure is related to this commit (a902058) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1396/builds/1590

Failed tests:

• test.test_multiprocessing_forkserver.test_processes

Failed subtests:

• test_repr_rlock - test.test_multiprocessing_forkserver.test_processes.WithProcessesTestLock.test_repr_rlock
• test_repr_rlock - test.test_multiprocessing_spawn.test_processes.WithProcessesTestLock.test_repr_rlock

Test leaking resources:

• test_processes: memory blocks

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.13.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/test/_test_multiprocessing.py", line 1492, in test_repr_rlock
    self.assertEqual('<RLock(SomeOtherThread, nonzero)>', repr(lock))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '<RLock(SomeOtherThread, nonzero)>' != '<RLock(None, 0)>'
- <RLock(SomeOtherThread, nonzero)>
+ <RLock(None, 0)>