Do return instead of crash on parasite not available

Do not trigger crash when checkpointed PID with
parasite was externally terminated, and
parasite PID was already reaped by Memcr service process.

memcr.c

@@ -2060,7 +2060,7 @@ static void *parasite_watch_thread(void *ptr)
 	ret = wait4(pid, &status, __WALL, NULL);
 	if (ret != pid) {
 		fprintf(stderr, "[-] wait4() ret %d != parasite %ld, errno %m\n", ret, pid);
-		assert(ret == pid);
+		return NULL;
 	}
 
 	parasite_status_signal(pid, status);