Unreachable code is apparently reachable in `linked_chunk::AsVector`

[Hywan]

We see this error in Sentry:

internal error: entered unreachable code: Inserting new chunk with no previous nor next chunk identifiers is impossible

It originates from:

matrix-rust-sdk/crates/matrix-sdk-common/src/linked_chunk/as_vector.rs

Lines 355 to 361 in 266abc0

	// Impossible state.
	(None, None) => {
	unreachable!(
	"Inserting new chunk with no previous nor next chunk identifiers \
	is impossible"
	);
	}

The code is receiving a new chunk, with no previous or next, but other chunks exist. This is impossible but we need to understand how it's possible to get in this situation. It seems related to the OrderTracker (again). The trace is the following: