ensure we're not dereferencing null pointers

[jimkring]

In this commit we added some code to toml_edit_item_get_type that checks if the item reference passed in from LabVIEW is null. If it is null, we don't try to deference it and just return a "None" value. We should do similar checks in other functions when receiving object references from LabVIEW before we de-reference them. If we don't, then we'll end up getting Error 1097 and/or crashing LabVIEW.

    ...
    // check if item has a null value
    if item.is_null() {
        let raw_string = match CString::new("None").unwrap().into_raw() {
            ptr if ptr.is_null() => {
                println!("Unable to allocate memory for string");
                return CString::new("").unwrap().into_raw();
            }
            ptr => ptr,
        };

        return raw_string;
    }

    // de-reference the item
    let item = unsafe { &mut *(item as *mut Item) };
    ...