RRC Reconfiguration leads in ASN1 message encoding failed (18446744073709551615)! after RRCReestablishment

This is not an easy Assertion to reproduce because most of the time the procedure succeeds.

Steps to reproduce:

1. A COTS UE triggers RRCReestablishmentRequest
2. gNB receives RRCReestablishmentRequest, stores spCellConfig in oldUE->CellGroup and drops for the current RNTI
3. gNB replies to the UE with RRCReestablishment
4. UE accepts RRCReestablishment but never sends RRCReestablishmentComplete to the gNB (yet undefined reason)
5. UE triggers new RA with Msg3 with MAC CE C-RNTI
6. gNB loses spCellConfig for the UE because it was never re-applied during the reestablishment procedures and the CellGroup message becomes corrupted.

/* 38.331 5.3.7.2 says that the UE releases the spCellConfig, so we drop it
     * from the current configuration. Also, expect the reconfiguration from
     * the CU, so save the old UE's CellGroup for the new UE */
    UE->CellGroup->spCellConfig = NULL;
    NR_UE_sched_ctrl_t *sched_ctrl = &UE->UE_sched_ctrl;
    NR_ServingCellConfigCommon_t *scc = mac->common_channels[0].ServingCellConfigCommon;
    configure_UE_BWP(mac, scc, sched_ctrl, NULL, UE, -1, -1);

    nr_mac_prepare_cellgroup_update(mac, UE, oldUE->CellGroup);
    oldUE->CellGroup = NULL;

Assertion (len > 0) failed!
In do_RRCReconfiguration() /home/user/openairinterface5g/openair2/RRC/NR/MESSAGES/asn1_msg.c:728
ASN1 message encoding failed (18446744073709551615)!

Exiting execution
/home/user/openairinterface5g/openair2/RRC/NR/MESSAGES/asn1_msg.c:728 do_RRCReconfiguration() Exiting OAI softmodem: _Assert_Exit_

RRC_Reconfiguration_leads_in_ASN1_message_encoding_failed__18446744073709551615_.txt