Make use of coverity

[jvoisin]

Because hardened_malloc is written in C (scary!) it would be nice to make use of coverity, if only to be on the safe side™. I would gladly do it, but because of the way github actions are working with secrets, and given how crappy coverity's ACL/interface is, @thestinger should do it.

Examples of integrations can be found here or here.

Of course, I'm happy to help :)