[TB] Strong protectors prevent deallocation even when they are zero-sized

[JoJoDeveloping]

Consider the following program:

fn protectme(_x: &mut (), y: *mut u8) {
    let y = unsafe { Box::from_raw(y) };
    drop(y)
}

fn main() {
    let y = Box::new(42u8);
    let y = Box::leak(y);
    let x = unsafe { &mut *(y as *mut u8 as *mut ()) };
    protectme(x, y);
}

The deallocation of the Box in protectme is currently reported as UB by Tree Borrows (but not by SB) because of the protector on _x. However, this protector is zero-sized (no offset have yet been accessed) so it's not actually protecting anything. It should not prevent the deallocation.

[JoJoDeveloping]

Turns out that #3831 indicates that this is intended behavior, but upon further reflection it does not make that much sense.