Fix a bug in the counter coroutine

[SnootierMoon]

This pull request fixes #98 and fixes #112.
The issue is that when counter happens to get called for the last time by run_hook, the coroutine terminates and returns nil, rather than returning true which is expected from coroutine.yield(true). Therefore, run_hook ends up returning nil here which causes counter to be called again in clone here. This resumes a dead coroutine, hence the error.
This PR fixes this by returning true at the end of the coroutine, which seems to be the intended behavior.

[savq]

Thanks!