program: Improve zero authority check

[febo]

Problem

While authority field on buffer and metadata accounts is Zeroable, allowing to remove the authority passing Some(Address::ZERO) is counterintuitive.

Solution

Add a check to prevent removing the authority by setting one as Address::ZERO.

[lorisleiva]

Interesting, could we not accept a regular Option<Address> instead of a Zeroable one?

[febo]

Interesting, could we not accept a regular Option<Address> instead of a Zeroable one?

We would need an extra byte on the account data. But I don't think it is too bad to have the extra check.

[lorisleiva]

I'm confused sorry. Address and ZeroableAddress have the same length right? If so, Option<Address> and Option<ZeroableAddress> should also have the same length (not matter how you serialise Option).