gh-125118: don't copy arbitrary values to _Bool in the struct module (GH-125169)

memcopy'ing arbitrary values to _Bool variable triggers undefined
behaviour. Avoid this.
We assume that `false` is represented by all zero bytes.

Credits to Alex Gaynor.

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>