gh-101037: Fix potential memory underallocation for zeros of int subtypes

[mdickinson]

This PR fixes object allocation in long_subtype_new to ensure that there's at least one digit in all cases, and makes sure that the value of that digit is copied over from the source long.

Needs backport to 3.11, but not any further: the change to require at least one digit was only introduced for Python 3.11.

Fixes#101037.

• Issue: long_subtype_new underallocates for zero #101037

[mdickinson]

@corona10 Would you be willing to review?

[mdickinson]

and makes sure that the value of that digit is copied over from the source long.

Just a comment: this seems fairly harmless in this case, but I'd note that we don't in general do anything special to make sure that ob_digit[0] contains anything meaningful in the case of zero being represented. (E.g., _PyLong_Copy won't copy over the zero digit.) I've added a note to longintrepr.h in this PR to clarify this.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @mdickinson for commit 2ddd5c1 🤖

If you want to schedule another build, you need to add the :hammer: test-with-buildbots label again.

[corona10]

@mdickinson I will take a look. Thanks

[corona10]

For refleak:

➜ cpython git:(pr/101038) ✗ ./python.exe -m test test_capi.test_misc -R 3:3
Raised RLIMIT_NOFILE: 256 -> 1024
0:00:00 load avg: 1.76 Run tests sequentially
0:00:00 load avg: 1.76 [1/1] test_capi.test_misc
beginning 6 repetitions
123456
......
test_capi.test_misc leaked [200, 200, 200] references, sum=600
test_capi.test_misc failed (reference leak)
== Tests result: FAILURE ==
1 test failed:
test_capi.test_misc
Total duration: 21.3 sec
Tests result: FAILURE

[corona10]

LGTM

The refleak is unrelated to this PR :)
see: #100998 (comment)

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit 877067c 🤖

If you want to schedule another build, you need to add the :hammer: test-with-refleak-buildbots label again.

[corona10]

@mdickinson Oops, sorry, I tried to rebase the branch for refleak test, but it messed up the commit history

[mdickinson]

@corona10 No problem; thanks for reviewing!

[miss-islington]

Thanks @mdickinson for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-101219 is a backport of this pull request to the 3.11 branch.