parasite-syscall: print an error message only in error cases

This patch suppresses these fake errors:

./socket-tcp6-local --pidfile=socket-tcp6-local.pid --outfile=socket-tcp6-local.out
Dump 5921
(00.041988) Error (parasite-syscall.c:388): si_code=1 si_pid=5936 si_status=0
(00.051210) Error (parasite-syscall.c:388): si_code=1 si_pid=5937 si_status=0
(00.070800) Error (parasite-syscall.c:388): si_code=1 si_pid=5938 si_status=0
(00.077582) Error (parasite-syscall.c:388): si_code=1 si_pid=5939 si_status=0
(00.091313) Error (action-scripts.c:59): One of more action scripts failed
Check results 5921
18:39:14.451:  5921: PASS
Test: zdtm/live/static/socket-tcp6-local, Result: PASS
ZDTM tests PASS.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

parasite-syscall.c

@@ -384,13 +384,13 @@ static void sigchld_handler(int signal, siginfo_t *siginfo, void *data)
 {
 	int pid, status;
 
-	pr_err("si_code=%d si_pid=%d si_status=%d\n",
-		siginfo->si_code, siginfo->si_pid, siginfo->si_status);
-
 	pid = waitpid(-1, &status, WNOHANG);
 	if (pid <= 0)
 		return;
 
+	pr_err("si_code=%d si_pid=%d si_status=%d\n",
+		siginfo->si_code, siginfo->si_pid, siginfo->si_status);
+
 	if (WIFEXITED(status))
 		pr_err("%d exited with %d unexpectedly\n", pid, WEXITSTATUS(status));
 	else if (WIFSIGNALED(status))