all: move defers to after error check to avoid nil indirection

Only affects some tests and none seem likely to be problematic, but let's fix them.
Fixes #3971.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6463060

src/pkg/net/http/transport_test.go

@@ -161,10 +161,10 @@ func TestTransportConnectionCloseOnResponse(t *testing.T) {
 				t.Fatalf("error in connectionClose=%v, req #%d, Do: %v", connectionClose, n, err)
 			}
 			body, err := ioutil.ReadAll(res.Body)
-			defer res.Body.Close()
 			if err != nil {
 				t.Fatalf("error in connectionClose=%v, req #%d, ReadAll: %v", connectionClose, n, err)
 			}
+			defer res.Body.Close()
 			return string(body)
 		}
 

src/pkg/os/os_test.go

@@ -69,10 +69,10 @@ var sysdir = func() (sd *sysDir) {
 
 func size(name string, t *testing.T) int64 {
 	file, err := Open(name)
-	defer file.Close()
 	if err != nil {
 		t.Fatal("open failed:", err)
 	}
+	defer file.Close()
 	var buf [100]byte
 	len := 0
 	for {
@@ -134,10 +134,10 @@ func TestStat(t *testing.T) {
 func TestFstat(t *testing.T) {
 	path := sfdir + "/" + sfname
 	file, err1 := Open(path)
-	defer file.Close()
 	if err1 != nil {
 		t.Fatal("open failed:", err1)
 	}
+	defer file.Close()
 	dir, err2 := file.Stat()
 	if err2 != nil {
 		t.Fatal("fstat failed:", err2)
@@ -189,10 +189,10 @@ func TestRead0(t *testing.T) {
 
 func testReaddirnames(dir string, contents []string, t *testing.T) {
 	file, err := Open(dir)
-	defer file.Close()
 	if err != nil {
 		t.Fatalf("open %q failed: %v", dir, err)
 	}
+	defer file.Close()
 	s, err2 := file.Readdirnames(-1)
 	if err2 != nil {
 		t.Fatalf("readdirnames %q failed: %v", dir, err2)
@@ -218,10 +218,10 @@ func testReaddirnames(dir string, contents []string, t *testing.T) {
 
 func testReaddir(dir string, contents []string, t *testing.T) {
 	file, err := Open(dir)
-	defer file.Close()
 	if err != nil {
 		t.Fatalf("open %q failed: %v", dir, err)
 	}
+	defer file.Close()
 	s, err2 := file.Readdir(-1)
 	if err2 != nil {
 		t.Fatalf("readdir %q failed: %v", dir, err2)
@@ -285,10 +285,10 @@ func TestReaddirnamesOneAtATime(t *testing.T) {
 		dir = "/bin"
 	}
 	file, err := Open(dir)
-	defer file.Close()
 	if err != nil {
 		t.Fatalf("open %q failed: %v", dir, err)
 	}
+	defer file.Close()
 	all, err1 := file.Readdirnames(-1)
 	if err1 != nil {
 		t.Fatalf("readdirnames %q failed: %v", dir, err1)