hostnamed: correct variable with errno in fallback_chassis
fixes assertion failure on arm:
systemd-hostnamed[642]: Assertion '(_error) != 0' failed at src/hostname/hostnamed.c:207, function fallback_chassis(). Aborting.
(cherry picked from commit 105a4245ff13d588e1e848e8ee3cffd6185bd0ae)
(cherry picked from commit 4a44597bdd725f504ebd520b0deef7797dc46daa)