Fix crashes due to access to deallocated memory



Crashes occur at FBDialog.m, lines 701 and 686 at attempt to detect 'respondsToSelector' in something like
     if ([_delegate respondsToSelector:@selector(dialogCompleteWithUrl:)]) {
            [_delegate dialogCompleteWithUrl:url];
    }

Fixed by zeroing delegate pointers when a delegate is being deallocated.