[BugFix][Runtime] Add missing check for PackedFunc

[cyx-6]

This pr fixs the missing check when assigning a PackedFunc into TVMRetValue.
For, example,

TVMRetValue value;
ObjectRef obj = some_packed_func;
value = obj; // Fails here
^^^^^^^^^^^
value.operator PackedFunc()

[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• cc @areusch _{See #10317 for details}

_{Generated by tvm-bot}