udf: Allow volume descriptor sequence to be terminated by unrecorded block

According to ECMA-167 3/8.4.2 a volume descriptor sequence can be
terminated also by an unrecorded block within the extent of volume
descriptor sequence. Currently we errored out in such case making such
volumes unmountable. Handle that case by treating any invalid block as a
block terminating the sequence.

Reported-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>