Added case for window_name_size == 0

Hello,

some programs (e. g. gimp, magick, ...) are able to export xwd files. However, they can't set the window name and as such, leave the field empty. xwud fails to display images where the win_name place doesn't even contain a null terminator (header_size == sizeof(xwd header) / win_name_size == 0).

I think this is nevertheless a valid case that might happen and want to propose this change that allows for displaying images without a window_name.