Fix allocation size for interface name to include null-byte

[kpcyrd]

Related to recent _FORTIFY_SOURCE=3 changes in Arch Linux:

https://gitlab.archlinux.org/archlinux/packaging/packages/sslh/-/issues/2

[stokito]

The resolve_listen function makes malloc itself internally.
So instead of the copying we can just assign directly.

[stokito]

Also the *listen = malloc(len * sizeof(**listen)); is probably better to replace with calloc

[yrutschle]

Thanks for the PR, even if I didn't use if :) I collected Sergey's comments.
I don't know how systemd-sslh-generator is called, so cannot test: please report if it causes problems.