service: don't report alien child as alive when it's not

When a sigchld is received from an alien child, main_pid is set to
0 then service_enter_running calls main_pid_good to check if the
child is running.  This incorrectly returned true because
kill(main_pid, 0) would return >= 0.

This fixes an error where a service would die and the cgroup would
become empty but the service would still report as active (running).

src/core/service.c

@@ -1865,7 +1865,7 @@ static int main_pid_good(Service *s) {
 
                 /* If it's an alien child let's check if it is still
                  * alive ... */
-                if (s->main_pid_alien)
+                if (s->main_pid_alien && s->main_pid > 0)
                         return kill(s->main_pid, 0) >= 0 || errno != ESRCH;
 
                 /* .. otherwise assume we'll get a SIGCHLD for it,