3bb0a4674f bench: Fix negative values and zero for -evals flag (nijynot)
Pull request description:
This PR makes `bench_bitcoin -evals=0` evaluate at once and throws when `-evals` is a negative integer.
---
Currently when you run `bench_bitcoin -evals=0`, it'll get stuck at
```
# Benchmark, evals, iterations, total, min, max, median
```
. This is not intuitively expected and should instead evaluate instantly as it's set to zero. Negative integers for `-evals` does not make sense either and should throw if set.
ACKs for top commit:
laanwj:
ACK 3bb0a4674f
Tree-SHA512: 03cd4c7c55134c7ffd8cdb6ee993551ce41061a73e13c3c047247af9df1fd7ed07d798272b643ec864099036922aaadbdcd2b798d710406f48df60b9d5448c26