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>

fs/udf/unicode.c

@@ -243,7 +243,7 @@ static int udf_name_from_CS0(struct super_block *sb,
 	}
 
 	if (translate) {
-		if (str_o_len <= 2 && str_o[0] == '.' &&
+		if (str_o_len > 0 && str_o_len <= 2 && str_o[0] == '.' &&
 		    (str_o_len == 1 || str_o[1] == '.'))
 			needsCRC = 1;
 		if (needsCRC) {