The exact semantics of `is_null` on unsized pointers are still debatable,
especially for trait objects. It may be legal to call `*mut self`
methods on a trait object someday, as with Go interfaces, so `is_null`
might need to validate the vtable pointer too.
For `as_ref` and `as_mut`, we're assuming that you cannot have a non-null
data pointer with a null vtable, so casting the unsized check is fine.
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