popen: fix access to freed memory after :close()

popen_delete() always frees a handle memory even when it reports a
failure to send SIGKILL, see [1]. We should reflect this contract in
popen_handle:close() and mark the handle as closed despite
popen_delete() return value.

There are cases, when killpg() fails with EPERM on Mac OS, so
popen_delete() reports a failure. See [1] for more information.

[1]: 01657bfb ('popen: always free
resources in popen_delete()')

Fixes #4995