Don't kill already-reaped process.

Don't kill already-reaped process.

* process.c (process_send_signal): Fix race condition where a
subprocess was reaped by a signal handler between the check for
liveness and calling 'kill', which meant that Emacs could in
theory kill an innocent bystander process.  Do the fix by blocking
SIGCHLD in a critical section that checks liveness before killing.

Fixes: debbugs:17561