Bluetooth: Controller: Segmentation fault in `isoal_rx_append_to_sdu`

[Thalley]

Describe the bug
After a BIS has been terminated, there is a change that isoal_rx_append_to_sdu is still called but where session->sdu_write is NULL, thus causing a segmentation fault when session->sdu_write is called unconditionally.

To Reproduce

1. Checkout tests: Bluetooth: Tester: BAP BSIM test #87207
2. export BOARD=nrf52_bsim/native
3. zephyr/tests/bsim/bluetooth/tester/compile.sh
4. zephyr/tests/bsim/bluetooth/tester/tests_scripts/bap_broadcast.sh
5. See error

d_01: @00:00:01.852878  [00:00:01.852,874] <err> bt_scan: bt_hci_le_biginfo_adv_report: Unknown handle 0x0000 for periodic advertising report
d_01: @00:00:01.852878  [00:00:01.852,874] <err> bt_iso: hci_iso: Unable to find conn for handle 7
p_2G4:d_11: @00:00:01.852950 ERROR: (/home/thalley/dev/nordic/zephyrproject/modules/bsim_hw_models/nrf_hw_models/src/HW_models/NHW_UART_backend_fifo.c:301): UART: Other end disconnected unexpectedly
 WARNING: (src/bs_pc_base.c:408): Device 1 left the party unsuspectingly.. I treat it as if it disconnected
d_11: @00:00:01.852950  TESTCASE NOT PASSED at exit (test return (1) indicates it was still in progress)
timeout: the monitored command dumped core

Expected behavior
Expect that terminating a BIS does not cause a fatal crash.

Impact
Significant issue and a blocker for #87207

Logs and console output
See above

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows): Arch Linux running Babblesim
• Toolchain (e.g Zephyr SDK, ...): Zephyr SDK
• Commit SHA or Version used: tests: Bluetooth: Tester: BAP BSIM test #87207 on top 6061deba555d148786ca2fd8193df7d6d6e1d0fcof

Additional context
May be related to #88370

[github-actions]

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[Thalley]

Still an issue and blocker today :)