Check length before assigning to a return tag (#606)

Author: tmcw

lib/infer/return.js

@@ -32,7 +32,7 @@ module.exports = function () {
       fn.returnType &&
       fn.returnType.typeAnnotation) {
       var returnType = flowDoctrine(fn.returnType.typeAnnotation);
-      if (comment.returns) {
+      if (comment.returns && comment.returns.length > 0) {
         comment.returns[0].type = returnType;
       } else {
         comment.returns = [{