tests: remove two misuses of nil pointers

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12858044

src/pkg/runtime/gc_test.go

@@ -136,7 +136,9 @@ func TestGcRescan(t *testing.T) {
 	for i := 0; i < 10; i++ {
 		p := &Y{}
 		p.c = make(chan error)
-		p.nextx = &head.X
+		if head != nil {
+			p.nextx = &head.X
+		}
 		p.nexty = head
 		p.p = new(int)
 		*p.p = 42