dot15d4: fix aux_sec_header incorrect parsing #8239

Job	Run time
Check SPDX identifiers	7s
Type hints check	29s
Build doc	8m 2s
pyca/cryptography test	35s
Code health check	1m 13s
CodeQL analysis	1m 9s
ubuntu-22.04 3.7 non_root -K scanner	4m 43s
ubuntu-latest 3.10 non_root -K scanner	4m 47s
ubuntu-latest 3.8 non_root -K scanner	4m 58s
ubuntu-latest 3.13 root -K scanner	5m 53s
ubuntu-latest pypy3.11 root -k scanner	3m 22s
ubuntu-latest 3.12 non_root -K scanner	5m 17s
ubuntu-latest libpcap 3.13 root -K scanner	6m 9s
ubuntu-latest 3.9 non_root -K scanner	5m 7s
macos-14 3.13 both -k scanner	1m 24s
ubuntu-latest 3.11 non_root -K scanner	4m 35s
ubuntu-latest 3.13 root -k scanner	20m 21s
macos-14 3.13 both -K scanner	4m 50s
windows-latest 3.13 root -K scanner	9m 56s
ubuntu-latest pypy3.11 root -K scanner	4m 57s
windows-latest 3.13 both -k scanner	5m 57s
	1h 43m 51s

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