Assert in OAI gNB when receiving DL traffic from DN and UE not connected anymore

To reproduce:

1. start OAI gNB in rfsimu mode
2. start OAI nrUE in rfsimu mode and connect to OAI gNB
3. start iperf TCP server on OAI nrUE
4. start iperf TCP in the DN and connect to OAI nrUE iperf TCP client
5. Let it flow for some time (with sustained traffic)
6. Disconnect the OAI nrUE (CTRL-C) while TCP traffic is being received.

The OAI gNB will exit with the following assertion:

Assertion (ret == HASH_TABLE_OK && data != ((void *)0)) failed!
In du_get_f1_ue_data() /home/oai/develop/openairinterface5g/openair2/F1AP/f1ap_ids.c:143
element for ue_id 14516 not found

Exiting execution
/home/oai/develop/openairinterface5g/openair2/F1AP/f1ap_ids.c:143 du_get_f1_ue_data() Exiting OAI softmodem: _Assert_Exit_
Aborted

The correct OAI gNB behavior should be to drop the DL packet possibly issuing an error log.