src: fix persistent handle lifecycle issue

Commit 636ca7c6 adds an optimization that casts strong Persistent<T>
handles directly to Local<T> handles to avoid the overhead of creating
new HandleScope-rooted Local<T> handles all the time.

One gotcha that I missed is that it's no longer legal to reference the
Local<T> after calling Persistent<T>::Dispose(). This commit addresses
that.