Add fuzzing for illegal variables and a bug in fuzzing code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.88235% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 96.09%. Comparing base (3f51342) to head (2bb3f39).

Files	Patch %	Lines
fuzz_test/fuzz_all_sat.cpp	36.84%	2 Missing and 22 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #36 +/- ## ========================================== - Coverage 98.63% 96.09% -2.55% ========================================== Files 24 27 +3 Lines 1538 1741 +203 Branches 1093 1213 +120 ========================================== + Hits 1517 1673 +156 - Misses 12 13 +1 - Partials 9 55 +46 ``` | [Flag](https://app.codecov.io/gh/michael-veksler/solver/pull/36/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Veksler) | Coverage Δ | | |---|---|---| | [Linux](https://app.codecov.io/gh/michael-veksler/solver/pull/36/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Veksler) | `60.86% <50.00%> (-1.45%)` | :arrow_down: | | [Windows](https://app.codecov.io/gh/michael-veksler/solver/pull/36/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Veksler) | `99.34% <100.00%> (+0.12%)` | :arrow_up: | | [macOS](https://app.codecov.io/gh/michael-veksler/solver/pull/36/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Veksler) | `35.90% <39.16%> (+0.35%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Veksler#carryforward-flags-in-the-pull-request-comment) to find out more.

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