Add non-NULL asserts to CMSG_* returns, null-initialize AncillaryBuf

[domenukk]

Added two checks for NULL returns and one zero-ing that is technically more correct

[tormol]

I've fixed the CI failure, please rebase.

[domenukk]

Rebased and switched to assert

[tormol]

Thanks!

[tormol]

Released as 0.2.7 and 0.3.0.