Found a failing `debug_assert` in MTUD

[p-avital]

Hi,

Our CI just caught a panic caused by this assertion.

I haven't started digging yet as to the severity, and it's the only case we've observed in a few months of development, so it doesn't look like the conditions that trigger it are likely; but I'm raising the issue in case it raises flags for someone :)

I'll try to investigate if I get a gap to do so, but if anyone familiar with the code could leave me some hints, I'd be grateful :)

[Ralith]

Thanks for the report! At a glance, I think the worst-case scenario is the peer closing the connection with a transport error if we send an MTU probe that exceeds the peer's UDP payload size limit, if MTUD is indeed running prematurely. In practice I expect most peers won't have restrictive payload size limits so this should be harmless, though it's still surprising and merits correction.

[zdave-parity]

Possibly fixed by #2400?

[Ralith]

Looks like; thanks!