nspawn: free D-Bus error before fallback (#39131)

nspawn: free D-Bus error before fallback (#39131)

Otherwise the next call fails:

Got message type=error sender=:1.5 destination=:1.470 path=n/a
    interface=n/a member=n/a cookie=151 reply_cookie=3 signature=s
    error-name=org.freedesktop.DBus.Error.UnknownMethod
    error-message=Unknown method CreateMachineEx or interface
    org.freedesktop.machine1.Manager. Assertion
'!bus_error_is_dirty(reterr_error)' failed at
    src/libsystemd/sd-bus/sd-bus.c:2396, function sd_bus_call(). Aborting.

Follow-up for d80af3b97b94e450a002ddac13874a953f0eab3e