Set up static analysis

Coverity Scan and Cppcheck look interesting. There's a nice comparison here. ecec is small, but the combination of crypto, string parsing, and C makes me nervous. 😁

This is a handy guide for humans writing C. It doesn't replace static analysis, of course, but I found it informative.

[adi-g15]

I tried DeepCode AI static analyzer on the repo, it has reported 13 issues (3 unique issues, repeated) ->

https://www.deepcode.ai/app/gh/web-push-libs/ecec/9c51ad6b959bc775b6a99018a43803d090ed9f05/_/dashboard