fix iterator traversal empty object panic (#314)

ast/iterator.go

@@ -63,7 +63,11 @@ func (self *Iterator) HasNext() bool {
     } else if self.p.t == _V_ARRAY_LAZY {
         return self.p.skipNextNode().Valid()
     } else if self.p.t == _V_OBJECT_LAZY {
-        return self.p.skipNextPair().Value.Valid()
+        pair := self.p.skipNextPair()
+        if pair == nil {
+            return false
+        }
+        return pair.Value.Valid()
     }
     return false
 }

ast/iterator_test.go

@@ -195,6 +195,16 @@ func TestIterator(t *testing.T) {
     if i != int64(loop) {
         t.Fatal(i)
     }
+
+    str, _ = getTestIteratorSample(0)
+    root, err = NewParser(str).Parse()
+    if err != 0 {
+        t.Fatal(err)
+    }
+    mi, _ = root.Get("object").Properties()
+    if mi.HasNext() {
+        t.Fatalf("should not have next")
+    }
 }
 
 func BenchmarkArrays(b *testing.B) {