We had this:

pub extern "C" fn rsvg_tree_free(tree: *mut RsvgTree) {
    if !tree.is_null() {
        let _ = unsafe { Box::from_raw(tree) };
                         ^ gets a Box<RsvgTree>, which is a box of a zero-sized enum

I.e. it frees zero bytes :)

With need this rebinding:

    let tree = unsafe { &mut *(tree as *mut Tree) };

i.e. cast tree as *mut Tree, so we'll end up with a Box<Tree>
which *can* be dropped as appropriate.

#325