Fixes #44, crash when no handlers

Recent database handler changes made the library crash, when feed
object is created without channel, item handlers (passing nils).
These simple changes seem to fix the problem and the library works
again.

feed.go

@@ -46,13 +46,17 @@ func (err *UnsupportedFeedError) Error() string {
 type ChannelHandlerFunc func(f *Feed, newchannels []*Channel)
 
 func (h ChannelHandlerFunc) ProcessChannels(f *Feed, newchannels []*Channel) {
-	h(f, newchannels)
+	if h != nil {
+		h(f, newchannels)
+	}
 }
 
 type ItemHandlerFunc func(f *Feed, ch *Channel, newitems []*Item)
 
 func (h ItemHandlerFunc) ProcessItems(f *Feed, ch *Channel, newitems []*Item) {
-	h(f, ch, newitems)
+	if h != nil {
+		h(f, ch, newitems)
+	}
 }
 
 type Handler interface {

feed_test.go

@@ -29,6 +29,15 @@ func TestFeed(t *testing.T) {
 	}
 }
 
+func Test_NoHandlers(t *testing.T) {
+	feed := New(1, true, nil, nil)
+	content, _ := ioutil.ReadFile("testdata/initial.atom")
+	err := feed.FetchBytes("http://example.com", content, nil)
+	if err != nil {
+		t.Error(err)
+	}
+}
+
 func Test_NewItem(t *testing.T) {
 	content, _ := ioutil.ReadFile("testdata/initial.atom")
 	feed := New(1, true, chanHandler, itemHandler)