bpo-37188: Fix a divide-by-zero in arrays of size-0 objects

[eric-wieser]

itemsize can be zero when _type_ is an empty struct or length-0 array.

This line used to be length * itemsize < 0, which didn't exhibit this problem.

https://bugs.python.org/issue37188

[eric-wieser]

@skrah, mind taking a look since this blocks #5576?

[bedevere-bot]

@skrah: Please replace # with GH- in the commit message next time. Thanks!

[miss-islington]

Thanks @eric-wieser for the PR, and @skrah for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7.
🐍🍒⛏🤖

[miss-islington]

Thanks @eric-wieser for the PR, and @skrah for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Sorry @eric-wieser and @skrah, I had trouble checking out the 2.7 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 0690c79c419b8d2bdfe7c5b6dca57b018f5a5a54 2.7

[bedevere-bot]

GH-13882 is a backport of this pull request to the 3.7 branch.

[skrah]

@eric-wieser Thanks for catching this! This appears to have been backported to 2.7 and the auto-backport fails. Do you have time to do a manual backport PR?

[skrah]

Actually I missed assert sizeof(obj) == 0, which should use self.assertEqual(). I can change that in a follow-up.

[miss-islington]

Thanks @eric-wieser for the PR, and @skrah for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7.
🐍🍒⛏🤖

[miss-islington]

Sorry, @eric-wieser and @skrah, I could not cleanly backport this to 2.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 0690c79c419b8d2bdfe7c5b6dca57b018f5a5a54 2.7

[bedevere-bot]

GH-13906 is a backport of this pull request to the 2.7 branch.

[miss-islington]

Thanks @eric-wieser for the PR, and @skrah for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-13911 is a backport of this pull request to the 3.8 branch.