udf: potential integer overflow

bloc->logicalBlockNum is unsigned so it's never less than zero.

When I saw that, it made me worry that "bloc->logicalBlockNum + count"
could overflow.  That's why I changed the check for less than zero
to an overflow check.  (The test works because "count" is also
unsigned.)

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>