bpo-30860: Fix deadcode in obmalloc.c

[vstinner]

Fix Coverity CID 1417587: _PyMem_Initialize() contains code which is
never executed.

https://bugs.python.org/issue30860

[vstinner]

I'm not sure why this change is needed since Include/internal/pymalloc.h contains:

#if NB_SMALL_SIZE_CLASSES > 64
#error "NB_SMALL_SIZE_CLASSES should be less than 64"
#endif /* NB_SMALL_SIZE_CLASSES > 64 */

So the code is already dead code? Maybe the code should be removed, but add a comment to Include/internal/pymalloc.h to ask to update _PyMem_Initialize() if NB_SMALL_SIZE_CLASSES becomes larger than 64?

[ericsnowcurrently]

So is coverity not smart enough to figure out the check across files? Or perhaps the coverity whitelist needs to be updated?

[vstinner]

I rewrote the PR to remove the deadcode. I added a build assertion as a reminder to update the code if NB_SMALL_SIZE_CLASSES is modified.