Correct wait4 when the child is not found

Make wait4 return -ECHILD when pid is specified and it's not found.

kernel/syscall.c

@@ -333,7 +333,7 @@ rescan:
 		ihk_mc_spinlock_unlock_noirq(&child_iter->lock);
 	}
 
-	if (empty) {
+	if (empty || (!child && pid != -1)) {
 		ihk_mc_spinlock_unlock_noirq(&proc->ftn->lock);	
 		return -ECHILD;
 	}