fix: prevent memory leak due to circular reference in vnodes

fix vuejs#6759

Author: yyx990803

src/core/instance/lifecycle.js

@@ -133,6 +133,10 @@ export function lifecycleMixin (Vue: Class<Component>) {
     if (vm.$el) {
       vm.$el.__vue__ = null
     }
+    // release circular reference (#6759)
+    if (vm.$vnode) {
+      vm.$vnode.parent = null
+    }
   }
 }