bench: Fix negative values and zero for -evals flag

Summary: This is a backport of Core PR17267

Test Plan:

$ ninja
$ src/bench/bitcoin-bench -evals -2
Error parsing command line arguments: Invalid parameter -2
$ echo $?
1
$ src/bench/bitcoin-bench -evals=0
$ echo $?
0

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8231