udf: Fix memory corruption when fs mounted with noadinicb option

When UDF filesystem is mounted with noadinicb mount option, it
happens that we extend an empty directory with a block. A code in
udf_add_entry() didn't count with this possibility and used
uninitialized data leading to memory and filesystem corruption.
Add a check whether file already has some extents before operating
on them.

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