sync/atomic: add Swap to nil deref test

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12870043

src/pkg/sync/atomic/atomic_test.go

@@ -1466,6 +1466,12 @@ func TestNilDeref(t *testing.T) {
 		func() { CompareAndSwapUint64(nil, 0, 0) },
 		func() { CompareAndSwapUintptr(nil, 0, 0) },
 		func() { CompareAndSwapPointer(nil, nil, nil) },
+		func() { SwapInt32(nil, 0) },
+		func() { SwapUint32(nil, 0) },
+		func() { SwapInt64(nil, 0) },
+		func() { SwapUint64(nil, 0) },
+		func() { SwapUintptr(nil, 0) },
+		func() { SwapPointer(nil, nil) },
 		func() { AddInt32(nil, 0) },
 		func() { AddUint32(nil, 0) },
 		func() { AddInt64(nil, 0) },