Uncatchable error is thrown if Contacts access is revoked from Settings

[qufighter]

An un-catchable error that causes application crash is thrown for me, on IOS 13.4 when after granting access to contacts, I revoke that access for the specific app from Settings screen.

https://github.com/jakeboxer/JBDeviceOwner/blob/master/JBDeviceOwner/JBDeviceOwner.m#L110

The following code corrects' the issue:

    if( addressBook != nil ){
        CFRelease(addressBook);
    }

Seems likely the nil reference is returned when the user has blocked access to the contacts. Maybe this is not an issue previously?

[jakeboxer]

Thanks for the heads-up @qufighter! Would you mind opening a PR for it? I'll accept and merge.