udf: Fix crash on IO error during truncate

When truncate(2) hits IO error when reading indirect extent block the
code just bugs with:

kernel BUG at linux-4.15.0/fs/udf/truncate.c:249!
...

Fix the problem by bailing out cleanly in case of IO error.

CC: stable@vger.kernel.org
Reported-by: jean-luc malet <jeanluc.malet@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>