Fuzzing failure

Running:
```
[dist]/verona/verona test --seed 1894715703
```
leads to 
```
Testing x100, seed: 1894715703
Testing pass: modules
Testing pass: structure
Testing pass: reference
Testing pass: conditionals
terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 18446744073709551615) >= this->size() (which is 0)
Aborted
```
