Fix nil dereference bug in entry checkAuthors

entry.go

@@ -36,6 +36,9 @@ type Entry struct {
 // atom:author element itself.
 func (e *Entry) checkAuthors() error {
 	if e.Authors == nil {
+		if e.Source == nil {
+			return errors.New("no authors set in entry")
+		}
 		if e.Source.Authors == nil {
 			return errors.New("no authors set in entry")
 		}