Fix EValue construction from a smart pointer. (#5357)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5357

The move constructor invoked by `*this =` expression tries to `destroy()` the `this` first, but it's not yet initialized.

For example, if the tag is randomly `Tag::ListTensor`, `destroy()` will try to iterate over and destroy a list of tensors. But since the memory is uninitialized, it will typically crash or corrupt memory.

Reviewed By: dbort

Differential Revision: D62658327

fbshipit-source-id: a12cb48f4bc3384a80534a453909e010c9549850