popen: remove retval from popen_stat()

The change 'popen: require popen handle to be non-NULL' makes
popen_stat() function always successful. There is no reason to return a
success / failure result.

See the previous similar patch: 'popen: remove retval from
popen_state()'.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>