Make a non-Py_DEBUG, asserts-enabled build of CPython possible. This means
making sure helper functions are defined when NDEBUG is not defined, not
just when Py_DEBUG is defined.

Also fix a division-by-zero in obmalloc.c that went unnoticed because in
Py_DEBUG mode, elsize is never zero.

(cherry picked from commit a00c3fd1 and 06bb4873)