[dev.garbage] runtime: fix TestLFStack on 386

LGTM=rlh
R=rlh, dvyukov
CC=golang-codereviews
https://golang.org/cl/157430044
diff --git a/src/runtime/lfstack_test.go b/src/runtime/lfstack_test.go
index e518777..68f221d 100644
--- a/src/runtime/lfstack_test.go
+++ b/src/runtime/lfstack_test.go
@@ -121,7 +121,7 @@
 			}
 			cnt++
 			sum2 += node.data
-			node.Next = nil
+			node.Next = 0
 		}
 	}
 	if cnt != K {