go.net/html: Fix comment handling for "in select" insertion mode

LGTM=andybalholm, nigeltao
R=golang-codereviews, gobot, nigeltao, andybalholm
CC=golang-codereviews
https://golang.org/cl/93680045

html/parse.go

@@ -1618,7 +1618,7 @@ func inSelectIM(p *parser) bool {
 			}
 		}
 	case CommentToken:
-		p.doc.AppendChild(&Node{
+		p.addChild(&Node{
 			Type: CommentNode,
 			Data: p.tok.Data,
 		})