New warnings in C implementation of subinterpreters

[sobolevn]

Bug report

See https://github.com/python/cpython/pull/134513/files

Check warning on line 3452 in Modules/_interpchannelsmodule.c
GitHub Actions / Cross build Linux
‘defaults.unboundop’ may be used uninitialized [-Wmaybe-uninitialized]

Check warning on line 3452 in Modules/_interpchannelsmodule.c
GitHub Actions / Cross build Linux
‘defaults.fallback’ may be used uninitialized [-Wmaybe-uninitialized]

CC @ericsnowcurrently

[ericsnowcurrently]

I've fixed the ones I introduced, afaik.