gh-112301: fix compiler warning about a possible use of an uninitialized variable

[carsonRadtke]

In #112301, @mdboom posted a proposal for adding some recommended compiler flags.

This PR addresses a single class of warnings (-Wmaybe-uninitialized) that may crop up if we were to switch to the "hardened" build command.

Specific build error:

./Modules/_io/fileio.c:175:9: warning: ‘exc’ may be used uninitialized [-Wmaybe-uninitialized]
  175 |         _PyErr_ChainExceptions1(exc);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Modules/_io/fileio.c: In function ‘_io_FileIO_close’:
./Modules/_io/fileio.c:160:15: note: ‘exc’ was declared here
  160 |     PyObject *exc;

• Issue: Consider applying flags for warnings about potential security issues #112301

[ghost]

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[sethmlarson]

Should we be adding the -Wmaybe-uninitialized option to the build in this PR as well? I think that's coming from -Wall but it's unlikely we enable that option in particular?

[mdboom]

Should we be adding the -Wmaybe-uninitialized option to the build in this PR as well? I think that's coming from -Wall but it's unlikely we enable that option in particular?

Alternatively, should we be adding -Werror=maybe-uninitialized which would error if this kind of thing ever crept back in? IMHO, warnings almost always just get ignored. I'm pretty comfortable that this warning is just a bunch of false positives that are hard to fix. On the other hand, we should probably build consensus before adding new error types. (Probably a discussion to have in the general sense over on #112301).

[vstinner]

The PyObject *exc = NULL; code was fixed by another change in the meanwhile: commit 05e4720.

I'm not sure that replacing if (res == NULL) { with if (exc != NULL) { is correct. IMO the current code is fine.

I close this PR since it's inactive since 2023: #112301 (comment).