plugins: handle corner case when PID file doesn't exist

If we get a notification and the service dies immediately, and also
removes its pid file, we need to take corrective action.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/pidfile.c

@@ -91,6 +91,11 @@ static void pidfile_update_conds(char *dir, char *name, uint32_t mask)
 			pid_t pid;
 
 			pid = pid_file_read(pid_file(svc));
+			if (pid == -1) {
+				cond_clear(cond);
+				return;
+			}
+
 			if (pid != svc->pid) {
 				_d("Forking service %s changed PID from %d to %d",
 				   svc->cmd, svc->pid, pid);