parse_file_list: handle files without an entry in the Files field

parse_file_list: handle files without an entry in the Files field

If a file is only listed in the Checksums-* fields, "entry" will be None and we
cannot call the get method to compare the size in the different fields.

This change just skips the comparison when "entry" is None. We can do this as
we later check that each entry has all required checksums (i.e. is listed in
Files and the Checksums-* fields).