udf: Fix lost indirect extent block

When inode ends with empty indirect extent block and we extended that
file, udf_do_extend_file() ended up just overwriting pointer to it with
another extent and thus effectively leaking the block and also
corruptiong length of allocation descriptors.

Fix the problem by properly following into next indirect extent when it
is present.

Signed-off-by: Jan Kara <jack@suse.cz>