Fix `dataoff` attribute access on `ext_fnhdr` #538

Job	Run time
unit - read only fixtures	22s
unit (registry.fedoraproject.org/fedora:38)	59s
unit (registry.fedoraproject.org/fedora:39)	1m 0s
unit (registry.fedoraproject.org/fedora:rawhide)	27s
unit (registry.opensuse.org/opensuse/leap:15.3)	1m 21s
unit (registry.opensuse.org/opensuse/leap:15.4)	1m 25s
unit (registry.opensuse.org/opensuse/leap:15.5)	1m 17s
unit (registry.opensuse.org/opensuse/tumbleweed)	59s
unit (quay.io/centos/centos:stream9)	34s
unit (docker.io/library/debian:stable)	28s
unit (docker.io/library/debian:unstable)	37s
unit (docker.io/library/ubuntu:latest)	28s
behave	17m 38s
	27m 35s

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