fix: check for nil link to avoid runtime panic gorilla#95 (gorilla#103)

## What type of PR is this? (check all applicable)
- [x] Bug Fix

## Description
Check for nil link in feed to avoid runtime panic

## Related Issue
- Closes gorilla#95

## Added/updated tests?
- [x] Yes

atom.go

@@ -89,13 +89,16 @@ type Atom struct {
 
 func newAtomEntry(i *Item) *AtomEntry {
 	id := i.Id
-
+	link := i.Link
+	if link == nil {
+		link = &Link{}
+	}
 	if len(id) == 0 {
 		// if there's no id set, try to create one, either from data or just a uuid
-		if len(i.Link.Href) > 0 && (!i.Created.IsZero() || !i.Updated.IsZero()) {
+		if len(link.Href) > 0 && (!i.Created.IsZero() || !i.Updated.IsZero()) {
 			dateStr := anyTimeFormat("2006-01-02", i.Updated, i.Created)
-			host, path := i.Link.Href, "/invalid.html"
-			if url, err := url.Parse(i.Link.Href); err == nil {
+			host, path := link.Href, "/invalid.html"
+			if url, err := url.Parse(link.Href); err == nil {
 				host, path = url.Host, url.Path
 			}
 			id = fmt.Sprintf("tag:%s,%s:%s", host, dateStr, path)
@@ -108,13 +111,13 @@ func newAtomEntry(i *Item) *AtomEntry {
 		name, email = i.Author.Name, i.Author.Email
 	}
 
-	link_rel := i.Link.Rel
+	link_rel := link.Rel
 	if link_rel == "" {
 		link_rel = "alternate"
 	}
 	x := &AtomEntry{
 		Title:   i.Title,
-		Links:   []AtomLink{{Href: i.Link.Href, Rel: link_rel, Type: i.Link.Type}},
+		Links:   []AtomLink{{Href: link.Href, Rel: link_rel, Type: link.Type}},
 		Id:      id,
 		Updated: anyTimeFormat(time.RFC3339, i.Updated, i.Created),
 	}
@@ -142,12 +145,16 @@ func newAtomEntry(i *Item) *AtomEntry {
 // create a new AtomFeed with a generic Feed struct's data
 func (a *Atom) AtomFeed() *AtomFeed {
 	updated := anyTimeFormat(time.RFC3339, a.Updated, a.Created)
+	link := a.Link
+	if link == nil {
+		link = &Link{}
+	}
 	feed := &AtomFeed{
 		Xmlns:    ns,
 		Title:    a.Title,
-		Link:     &AtomLink{Href: a.Link.Href, Rel: a.Link.Rel},
+		Link:     &AtomLink{Href: link.Href, Rel: link.Rel},
 		Subtitle: a.Description,
-		Id:       a.Link.Href,
+		Id:       link.Href,
 		Updated:  updated,
 		Rights:   a.Copyright,
 	}

feed_test.go

@@ -504,6 +504,58 @@ func TestFeedSorted(t *testing.T) {
 	}
 }
 
+func TestFeedNil(t *testing.T) {
+	now, err := time.Parse(time.RFC3339, "2013-01-16T21:52:35-05:00")
+	if err != nil {
+		t.Error(err)
+	}
+	tz := time.FixedZone("EST", -5*60*60)
+	now = now.In(tz)
+
+	feed := &Feed{
+		Title:       "jmoiron.net blog",
+		Link:        nil,
+		Description: "discussion about tech, footie, photos",
+		Author:      nil,
+		Created:     now,
+		Copyright:   "This work is copyright © Benjamin Button",
+	}
+
+	feed.Items = []*Item{
+		{
+			Title:       "Limiting Concurrency in Go",
+			Link:        nil,
+			Description: "A discussion on controlled parallelism in golang",
+			Author:      nil,
+			Created:     now,
+			Content:     `<p>Go's goroutines make it easy to make <a href="http://collectiveidea.com/blog/archives/2012/12/03/playing-with-go-embarrassingly-parallel-scripts/">embarrassingly parallel programs</a>, but in many &quot;real world&quot; cases resources can be limited and attempting to do everything at once can exhaust your access to them.</p>`,
+		}}
+
+	if _, err := feed.ToAtom(); err != nil {
+		t.Errorf("unexpected error encoding Atom: %v", err)
+	}
+	var buf bytes.Buffer
+	if err := feed.WriteAtom(&buf); err != nil {
+		t.Errorf("unexpected error writing Atom: %v", err)
+	}
+
+	if _, err := feed.ToRss(); err != nil {
+		t.Errorf("unexpected error encoding RSS: %v", err)
+	}
+	buf.Reset()
+	if err := feed.WriteRss(&buf); err != nil {
+		t.Errorf("unexpected error writing RSS: %v", err)
+	}
+
+	if _, err := feed.ToJSON(); err != nil {
+		t.Errorf("unexpected error encoding JSON: %v", err)
+	}
+	buf.Reset()
+	if err := feed.WriteJSON(&buf); err != nil {
+		t.Errorf("unexpected error writing JSON: %v", err)
+	}
+}
+
 var jsonOutputHub = `{
   "version": "https://jsonfeed.org/version/1",
   "title": "feed title",

rss.go

@@ -145,9 +145,13 @@ func (r *Rss) RssFeed() *RssFeed {
 		image = &RssImage{Url: r.Image.Url, Title: r.Image.Title, Link: r.Image.Link, Width: r.Image.Width, Height: r.Image.Height}
 	}
 
+	var href string
+	if r.Link != nil {
+		href = r.Link.Href
+	}
 	channel := &RssFeed{
 		Title:          r.Title,
-		Link:           r.Link.Href,
+		Link:           href,
 		Description:    r.Description,
 		ManagingEditor: author,
 		PubDate:        pub,