validate/CheckRoot: fix panic on empty spec

Fix the following panic:

> panic: runtime error: invalid memory address or nil pointer dereference [recovered]
> 	panic: runtime error: invalid memory address or nil pointer dereference
> ...
> github.com/opencontainers/runtime-tools/validate.(*Validator).CheckRoot(0xc00049b748)
> 	/home/kir/go/src/github.com/opencontainers/runtime-tools/validate/validate.go:192 +0x278

This is caused by empty Windows field, which is a spec violation, but
it should not result in a nil pointer dereference

Add a test case (which fails before the fix).

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

validate/validate.go

@@ -170,8 +170,8 @@ func (v *Validator) CheckJSONSchema() (errs error) {
 func (v *Validator) CheckRoot() (errs error) {
 	logrus.Debugf("check root")
 
-	if v.platform == "windows" && v.spec.Windows != nil {
-		if v.spec.Windows.HyperV != nil {
+	if v.platform == "windows" {
+		if v.spec.Windows != nil && v.spec.Windows.HyperV != nil {
 			if v.spec.Root != nil {
 				errs = multierror.Append(errs,
 					specerror.NewError(specerror.RootOnHyperVNotSet, fmt.Errorf("for Hyper-V containers, Root must not be set"), rspec.Version))
@@ -182,7 +182,7 @@ func (v *Validator) CheckRoot() (errs error) {
 				specerror.NewError(specerror.RootOnWindowsRequired, fmt.Errorf("on Windows, for Windows Server Containers, Root is REQUIRED"), rspec.Version))
 			return
 		}
-	} else if v.platform != "windows" && v.spec.Root == nil {
+	} else if v.spec.Root == nil {
 		errs = multierror.Append(errs,
 			specerror.NewError(specerror.RootOnNonWindowsRequired, fmt.Errorf("on all other platforms, Root is REQUIRED"), rspec.Version))
 		return

validate/validate_test.go

@@ -289,6 +289,7 @@ func TestCheckRoot(t *testing.T) {
 		{rspec.Spec{Windows: &rspec.Windows{HyperV: &rspec.WindowsHyperV{}}, Root: nil}, "windows", specerror.NonError},
 		{rspec.Spec{Windows: &rspec.Windows{}, Root: &rspec.Root{Path: filepath.Join(tmpBundle, "rootfs")}}, "windows", specerror.RootPathOnWindowsGUID},
 		{rspec.Spec{Windows: &rspec.Windows{}, Root: &rspec.Root{Path: "\\\\?\\Volume{ec84d99e-3f02-11e7-ac6c-00155d7682cf}\\"}}, "windows", specerror.NonError},
+		{rspec.Spec{}, "windows", specerror.RootOnWindowsRequired},
 		{rspec.Spec{Windows: &rspec.Windows{}, Root: nil}, "windows", specerror.RootOnWindowsRequired},
 		{rspec.Spec{Root: nil}, "linux", specerror.RootOnNonWindowsRequired},
 		{rspec.Spec{Root: &rspec.Root{Path: "maverick-rootfs"}}, "linux", specerror.RootPathOnPosixConvention},