Fix nil checks for vsm.Attach*() methods.

internal/pkg/vsm/e2e_test.go

@@ -143,6 +143,15 @@ func TestAttachInstance(t *testing.T) {
 		t.Error("expected AttachInstance() to fail for a " +
 			"non-existent instance")
 	}
+
+	var n *VSM
+	if err := n.AttachInstance("gotest", false); err == nil {
+		t.Error("expected nil.AttachInstance() to fail")
+	}
+	uninit := new(VSM)
+	if err := uninit.AttachInstance("gotest", false); err == nil {
+		t.Error("expected uninitialized.AttachInstance() to fail")
+	}
 }
 
 func TestPointer(t *testing.T) {

internal/pkg/vsm/vsm.go

@@ -139,9 +139,6 @@ func (v *VSM) AttachTmo(tmo time.Duration) error {
 }
 
 func (v *VSM) attach(progress bool) error {
-	if err := v.checkNil(); err != nil {
-		return err
-	}
 	prog := -1
 	if progress {
 		prog = 0
@@ -156,12 +153,18 @@ func (v *VSM) attach(progress bool) error {
 // Attach attaches to the default instance of Varnish, as documented
 // for the Attacher interface.
 func (v *VSM) Attach(progress bool) error {
+	if err := v.checkNil(); err != nil {
+		return err
+	}
 	return v.attach(progress)
 }
 
 // AttachInstance attaches to a named instance of Varnish, as
 // documented for the Attacher interface.
 func (v *VSM) AttachInstance(inst string, progress bool) error {
+	if err := v.checkNil(); err != nil {
+		return err
+	}
 	C.VSM_ResetError(v.vsm)
 	if C.VSM_Arg(v.vsm, 'n', C.CString(inst)) != 1 {
 		return v