ldns-read-zone -u fails if a type is the only type in a window and the type modulo 256 is equal to zero

[Philip-NLnetLabs]

Assuming there is a type with value 65280 then

examples/ldns-read-zone -u TYPE65280
Cannot set rr type (null) in output format to print as unknown type: The RR type bitmap rdata field did not have a bit reserved for the specific RR type

This fails because ldns_rdf_bitmap_known_rr_types_set does not allocate window 255.