Add missing condition to stop reaping child procs

[jprendes]

Since PR #75 the loop that reaps child processes is not properly stopping.
Since WaitPidFlag::WNOHANG is being used, the WaitStatus::StillAlive status should also be considered.

StillAlive

There are currently no state changes to report in any awaited child process. This is only returned if WaitPidFlag::WNOHANG was used (otherwise wait() or waitpid() would block until there was something to report).

This is equivalent to the res_pid == 0 condition before PR #75.

[jprendes]

The CI failure seems like an unrelated transient network issue

+ integration
go: google.golang.org/genproto@v0.0.0-20230306155012-7f2fa6fef1f4 requires
	cloud.google.com/go/bigquery@v1.48.0 requires
	github.com/apache/arrow/go/v10@v10.0.1 requires
	lukechampine.com/uint[12](https://github.com/containerd/rust-extensions/actions/runs/5466464185/jobs/9961489697?pr=151#step:7:13)8@v1.2.0: unrecognized import path "lukechampine.com/uint128": https fetch: Get "https://lukechampine.com/uint128?go-get=1": dial tcp [2600:3c03::f03c:91ff:fe55:92ba]:443: connect: network is unreachable
make: *** [Makefile:215: integration] Error 1
Error: Process completed with exit code 2.