PYTHON-5571 - Fix memory leak when raising InvalidDocument with C extensions

[NoahStapp]

We previously redeclared msg, new_msg, and new_evalue. This shadowed the initial declarations and leaked memory at cleanup. This PR removes that shadowing and transfers ownership of new_evalue's reference correctly.

[blink1073]

LGTM!

[blink1073]

drivers-pr-bot please backport to v4.15

[mongodb-dbx-release-bot]

Sorry, unable to cherry-pick to v4.15, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout -b cherry-pick-v4.15-52400e11a1ac69bcc89dfe5a034df00a4a7f690c v4.15

git fetch origin 52400e11a1ac69bcc89dfe5a034df00a4a7f690c

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 52400e11a1ac69bcc89dfe5a034df00a4a7f690c

3. You will likely have some merge/cherry-pick conflicts here, fix them and commit:

git commit -am {message}

4. Push to a named branch:

git push origin cherry-pick-v4.15-52400e11a1ac69bcc89dfe5a034df00a4a7f690c

5. Create a PR against branch v4.15. I would have named this PR:

"PYTHON-5571 - Fix memory leak when raising InvalidDocument with C extensions (#2573) [v4.15]"