Fix nested choice when no struct/set present

[Nicceboy]

Fixes #168

[Nicceboy]

Build seems to fail for third party issue:

  = note: /usr/local/lib/gcc/x86_64-unknown-netbsd/9.4.0/../../../../x86_64-unknown-netbsd/bin/ld: cannot find -lexecinfo
          collect2: error: ld returned 1 exit status
          

error: could not compile `rasn-compiler` (bin "rasn-compiler") due to previous error
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.

[XAMPPRocky]

Thank you for your PR! I think the question I have is, is why is the variance wrong? If there's two variants, the variance should be two, not three. This PR solves this case, but doesn't really answer the root issue.

[Nicceboy]

The variance is correct but the range for constraint is incorrectly set because index starts from zero (classic).

[XAMPPRocky]

Ah gotcha, would you be able to make this a function? Having - 1 peppered in the code is a bit of a smell as it's not obvious just by looking at it why, so it could be forgotten when the code is revisited.

[Nicceboy]

I added separate variable declaration and some comments. Also debug asserts that variance length should be always greater than 0, which in theory should be always greater, but no other checks. (You cannot pass variant if enum does not have one...).
Would that be okay?

[XAMPPRocky]

Thank you for your PR!