udf: Fix handling of Partition Descriptors

Current handling of Partition Descriptors in Volume Descriptor Sequence
is buggy in several ways. Firstly, it does not take descriptor sequence
numbers into account at all, thus any volume making serious use of them
would be unmountable. Secondly, it does not handle Volume Descriptor
Pointers or Volume Descriptor Sequence without Terminating Descriptor.

Fix these problems by properly remembering all Partition Descriptors in
the Volume Descriptor Sequence and their sequence numbers. This is made
more complicated by the fact that we don't know number of partitions in
advance and sequence numbers have to be tracked on per-partition basis.

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