Fix an issue with referencing of a map in ctl.ImportSubnet

[errordeveloper]

Description

This fixes #518, which is a bug that got introduced as part of #513.

Checklist

• Code compiles correctly (i.e make build)
• All unit tests passing (i.e. make test)
• All integration tests passing (i.e. make integration-test)

[mumoshu]

Just trying to understand what was going on here.

The cause of the issue seems to be https://github.com/weaveworks/eksctl/pull/513/files#diff-9cf0956b5581521f18ebe4dedc1f1552L90 - we had been mistakenly throwing away the imported subnets when c.VPC.Subnets.Private or c.VPC.Subnets.Publicwas empty?

[errordeveloper]

Thanks for asking! I've looked into this some more, and actually have a better idea of what is going on.

I was assuming that maps are always treated as references, which is indeed the case. However, if you assign a nil map to another nil map, nothing will actually happen. Namely, maps are treated as references (well, sort of), but only after they were initialised. Take a look here: https://play.golang.org/p/OTCUWDoH9HK. I didn't realise this until now, or at least I forgot this.

[errordeveloper]

Actually, it's even simpler than this, any nil reference can be assigned to another one, but nothing gets copied really: https://play.golang.org/p/KFefMZEabWQ.

[errordeveloper]

So it sort of boils down to i1 = i2 being the same as i1 = nil, so the compiler cannot really tell these apart, hence it has no way of telling me that I'm probably doing something wrong... TIL :)