SSUSession: get either IPv4/6 intro key from remote

[anonimal]

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Resolves #997 (should).

As for the need to throw for all null deference checks, I'll provide a better solution when I return so we don't have to litter the code with throws for every pointer.

[anonimal]

returns a nullptr, a null-dereference can still happen in release builds.

It's not possible because nullptr should never be returned because a transport address will be available (by that point).

Looking forward to your solution for null pointers.

The solution is not for null pointers but for how we assert and throw.