Add some checks for NULL for mempcy and its friends

[jvoisin]

See:

• https://www.imperialviolet.org/2016/06/26/nonnull.html
• https://davidben.net/2024/01/15/empty-slices.html
• https://stackoverflow.com/questions/5243012/is-it-guaranteed-to-be-safe-to-perform-memcpy0-0-0

Thanks to @gendx for telling me about this :)

[jvoisin]

Done via 140cffb