Previously, `channelpadding_get_netflow_inactive_timeout_ms` would
crash with an assertion failure if `low_timeout` was greater than
`high_timeout`. That wasn't possible in practice because of checks
in `channelpadding_update_padding_for_channel`, but it's better not
to have a function whose correctness is this tricky to prove.
Fixes#40645. Bugfix on 0.3.1.1-alpha.
