libblkid: iso9660: check that iso->publisher_id and iso->application_id are not file...

libblkid: iso9660: check that iso->publisher_id and iso->application_id are not file paths

ECMA 119 (ISO 9660) says that if the first byte is set to (5F), the
remaining bytes of this field shall specify an identifier for a file
containing the data. As libblkid does not support reading files from the
filesystem itself it cannot parse fields which starts with 0x5F '_' char.

So for now ignore iso->publisher_id and iso->application_id values which
starts with '_' character.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>