udf: Fix uninitialized array access for some pathnames

For filenames that begin with . and are between 2 and 5 characters long,
UDF charset conversion code would read uninitialized memory in the
output buffer. The only practical impact is that the name may be prepended a
"unification hash" when it is not actually needed but still it is good
to fix this.

Reported-by:  <syzbot+cd311b1e43cc25f90d18@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/000000000000e2638a05fe9dc8f9@google.com
Signed-off-by: Jan Kara <jack@suse.cz>