host/ble_l2cap_coc: fix possible race condition in L2CAP COC

[KKopyscinski]

Two functions: ble_l2cap_coc_continue_tx and ble_l2cap_coc_recv_ready can be reached from other task than host, for example separate thread running in application, by calling ble_l2cap_recv_ready or ble_l2cap_send. Because pointer to chan passed into these functions may be outdated (connection is being terminated by host) this will lead to either:

• triggering BLE_HS_DBG_ASSERT(conn != NULL) in ble_hs_conn_find_assert
• dereferencing NULL pointer to conn, after ble_hs_conn_find_assert returns NULL (case with disabled BLE_HS_DEBUG) These two cases shall not cause assert, and should be prepared for chan not existing anymore. Now, BLE_HS_ENOTCONN is returned if connection no longer exists.

[KKopyscinski]

Rest of functions that use ble_hs_conn_find_assert are reachable from host and should indeed cause assert to be triggered, or have conn already checked, as described in 8ec1b8d BLE Host - Check conn state once before proc.