bluetooth: ots: Enable testing with assert

Fix compilation and logic issues to allow testing OTS functionality with
assert enabled.

The compilation fixes are simple and do not require an explanation.

The assertion in bt_ots_dir_list_init was logically reversed. Its
purpose is to ensure the directory list is not already initialized
and as such should assert that the dir_list is not set (i.e. is NULL).

Signed-off-by: Abe Kohandel <abe.kohandel@gmail.com>