Missing BVCI in FLUSH_LL_ACK can cause segfault
The BVCI IE is listed as conditional and is only included if the flush action indicates that LLC-PDUs are transferred. (3GPP TS 48.018 Ch. 10.4.2).
The code in gbprox_rx_sig_from_bss unconditionally tries to get a BVCI from a FLUSH_LL message which could result in a segfault if no such IE is included.