Bluetooth: Host: Check that `conn` is not NULL

In `gatt.c`, the function `bt_gatt_ccc_cfg_conn_lookup()` was not
checking that `conn` was not NULL. That leaded to a NULL pointer
dereferences later in `bt_conn_is_peer_addr_le`.

Fix by checking that `conn` is not NULL.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>