Fix a check, make a netflow padding function more safe.

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 (closed). Bugfix on 0.3.1.1-alpha.

I've made this patch against 0.4.5 in case we decide to Backport.