get_length16

# File lib/resolv.rb, line 1304
        def get_length16
          len, = self.get_unpack('n')
          save_limit = @limit
          @limit = @index + len
          d = yield(len)
          if @index < @limit
            raise DecodeError.new("junk exists")
          elsif @limit < @index
            raise DecodeError.new("limit exceeded")
          end
          @limit = save_limit
          return d
        end