agent: Simplify agent_popup_message_stop.
* agent/call-pinentry.c (agent_popup_message_stop): Just kill it.
By checking if it's alive or not, we can lower a risk of sending
SIGINT to a wrong process on unusual condition when PID is re-used to
a different process.
That's true, however, since it's alive usually, simply sending SIGINT
is enough here.
Note that here is a race condition for detecting if process is active
or not; A process can die just after being detected alive.
Moreover, when the process of pinentry accidentally died already, it
should have caused return of assuan_transact and the thread of
popup_message_thread likely already set popup_finished=1.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>