Our own implementation ignores NULL values, however, explicit_bzero()
can't handle that, as indicated by the `__nonnull ((1))` attribute in the
function's signature in string.h, and causes a segmentation fault. This
was noticed in one of the unit tests for NewHope. Since we usually use
memwipe() via chunk_clear(), which already ignores NULL pointers, this
is not that much of an issue in practice.
Fixes: 149d1bbb05 ("memory: Use explicit_bzero() as memwipe() if available")
cf98706bb8