Fails self-test when built with `-D_FORTIFY_SOURCE=3`, implying buffer overflows

[risicle]

NixOS now builds all packages with -D_FORTIFY_SOURCE=3. This causes the hash_extender --test mode to fail with *** buffer overflow detected ***: terminated. This is on x86_64 linux using gcc 12.

See full build log @ https://hydra.nixos.org/log/ilpbzm1cw05p7z10ma5zlrxdgx4s2kc1-hash_extender-unstable-2020-03-24.drv (mirror @ https://gist.github.com/risicle/2a24820ff99ea7d63b8cc5f1215f4027)