udf: Don't BUG on missing metadata partition descriptor

Currently, if a metadata partition map is missing its partition descriptor,
then udf_get_pblock_meta25() will BUG() out the first time it is called.
This is rather drastic for a corrupted filesystem, so just treat this case
as an invalid mapping instead.

Signed-off-by: Alden Tondettar <alden.tondettar@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>