renderer: Don't panic for unknown NodeKind

When rendering a Node, Renderer looks up the NodeRenderer for that
node's kind in a slice. That will panic with an "index out of range"
error if a node renderer for that node kind has not been registered.

This change verifies that a node renderer is available for that node
kind before attempting to use it, and instead of panicking, returns an
informative error message.

renderer/renderer.go

@@ -3,6 +3,7 @@ package renderer
 
 import (
 	"bufio"
+	"fmt"
 	"io"
 	"sync"
 
@@ -161,7 +162,14 @@ func (r *renderer) Render(w io.Writer, source []byte, n ast.Node) error {
 	err := ast.Walk(n, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		s := ast.WalkStatus(ast.WalkContinue)
 		var err error
-		f := r.nodeRendererFuncs[n.Kind()]
+
+		k := n.Kind()
+		if int(k) >= len(r.nodeRendererFuncs) {
+			return s, fmt.Errorf("unrecognized node kind %v cannot be rendered: "+
+				"register a renderer for this node kind first", k)
+		}
+
+		f := r.nodeRendererFuncs[k]
 		if f != nil {
 			s, err = f(writer, source, n, entering)
 		}

renderer/renderer_test.go

@@ -0,0 +1,25 @@
+package renderer
+
+import (
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+)
+
+func TestRenderUnknownNode(t *testing.T) {
+	src := []byte("# Foo\n\nHello world")
+	node := parser.NewParser().Parse(text.NewReader(src))
+
+	r := NewRenderer()
+	err := r.Render(ioutil.Discard, src, node)
+	if err == nil {
+		t.Fatalf("Render() expected error")
+	}
+
+	if !strings.Contains(err.Error(), "unrecognized node kind Document") {
+		t.Errorf("Render() failed with unexpected error: %v", err)
+	}
+}