Avoid race condition in target list init

As reported by Go's race detector.

See #917

hugolib/site.go

@@ -77,6 +77,7 @@ type Site struct {
 	Menus          Menus
 	timer          *nitro.B
 	Targets        targetList
+	targetListInit sync.Once
 	Completed      chan bool
 	RunMode        runmode
 	params         map[string]interface{}
@@ -1454,32 +1455,39 @@ func (s *Site) NewXMLBuffer() *bytes.Buffer {
 }
 
 func (s *Site) PageTarget() target.Output {
+	s.initTargetList()
+	return s.Targets.Page
+}
+
+func (s *Site) FileTarget() target.Output {
+	s.initTargetList()
+	return s.Targets.File
+}
+
+func (s *Site) AliasTarget() target.AliasPublisher {
+	s.initTargetList()
+	return s.Targets.Alias
+}
+
+func (s *Site) initTargetList() {
+	s.targetListInit.Do(func() {
 		if s.Targets.Page == nil {
 			s.Targets.Page = &target.PagePub{
 				PublishDir: s.absPublishDir(),
 				UglyUrls:   viper.GetBool("UglyUrls"),
 			}
 		}
-	return s.Targets.Page
-}
-
-func (s *Site) FileTarget() target.Output {
 		if s.Targets.File == nil {
 			s.Targets.File = &target.Filesystem{
 				PublishDir: s.absPublishDir(),
 			}
 		}
-	return s.Targets.File
-}
-
-func (s *Site) AliasTarget() target.AliasPublisher {
 		if s.Targets.Alias == nil {
 			s.Targets.Alias = &target.HTMLRedirectAlias{
 				PublishDir: s.absPublishDir(),
 			}
-
 		}
-	return s.Targets.Alias
+	})
 }
 
 func (s *Site) WriteDestFile(path string, reader io.Reader) (err error) {