zhack is flaky on FreeBSD

Describe the problem you're observing

zhack occasionally segfaults on FreeBSD, causing some tests to fail randomly. When I looked at the core in a debugger, the fault was literally in rand(), but it really doesn't make sense to me. I suspect somewhere else is corrupting the stack.

Describe how to reproduce the problem

Run a test that uses zhack, such as cli_root/zpool_import/zpool_import_features_001_pos, on FreeBSD until it fails.

Include any warning/errors/backtraces from the system logs