_struct.c: Fix UB from integer overflow in prepare_s

[StanFromIreland]

Caught by OSS-Fuzz, see https://issues.oss-fuzz.com/issues/466669135.

stan@stanlaptop:~/dev/cpython{main}$ ./python -c 'import struct; struct.Struct("6107082938247334i9221070829382473344q")'
Modules/_struct.c:1681:26: runtime error: signed integer overflow: 6107082938247334 + 9221070829382473344 cannot be represented in type 'long int'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import struct; struct.Struct("6107082938247334i9221070829382473344q")
                   ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: total struct size too long

Skipping news, since there shouldn't be any user-visible change, although I can add one if people prefer.

[gpshead]

I suggest adding a one line news entry.

[StanFromIreland]

I suggest adding a one line news entry.

Done, I used the PR number instead of creating an issue.

[miss-islington-app]

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

[bedevere-app]

GH-145162 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-145163 is a backport of this pull request to the 3.13 branch.