C: clamping of `budget` in `elements_simplicity_execSimplicity` is incorrect

[apoelstra]

In elements_simplicity_execSimplicity we clamp the passed budget value to ensure it does not exceed BUDGET_MAX. But we do not check for negative values. If you pass a negative value, it will be casted to a ubounded that exceeds BUDGET_MAX, eventually triggering an assertion failure in eval.c.

There are three ways we might handle negative values:

• clamping to 0
• returning an error
• asserting in execSimplicity

I have no preference about which.

[roconnor-blockstream]

I'm leaning towards making the budget non-negative a precondition and adding an assertion in execSimplicity.

[roconnor-blockstream]

Fixed in 'staging'.