Out of bounds panic in VacantEntry.insert

I received a [report](https://github.com/greatest-ape/aquatic/issues/126) of a panic occurring [in VacantEntry.insert](https://github.com/jonhoo/indexmap-amortized/blob/7902615cf1d38927e2e3d7bf5069c59af458c672/src/map/core.rs#L615).

I unfortunately don't have a minimal code sample, but it is likely triggered by this line: https://github.com/greatest-ape/aquatic/blob/bdf200019ad3b6bb93dd18227b583237b9f188f7/aquatic_udp/src/workers/swarm/mod.rs#L148

Start with VacantEntry.insert in src/map/core.rs at the linked line, then inspect the aquatic_udp swarm caller at aquatic_udp/src/workers/swarm/mod.rs:148. Reproduce the reported out-of-bounds panic despite the missing minimal example, identify the triggering state, and verify that it no longer panics with a regression test.

Written by the indexing model from the issue text.