This fixes#4436 and possible other unnoticed crashes by adding a defensive check.
Reviewed-on: https://git.minetest.land/VoxeLibre/VoxeLibre/pulls/4437
Reviewed-by: Mikita Wiśniewski <rudzik8@protonmail.com>
Co-authored-by: the-real-herowl <wiktor_t-i@proton.me>
Co-committed-by: the-real-herowl <wiktor_t-i@proton.me>