Server Crash - runtime error: index out of range

*This issue was originally created by [**@hugeblank**](https://github.com/hugeblank) at .*

### Background information

- **Dendrite version or git SHA**: 3d02c8103152fbb9a5401a091ef0f681a32bfeee
- **SQLite3 or Postgres?**: postgres (PostgreSQL) 15.4
- **Running in Docker?**: No
- **`go version`**: `go version go1.21.1 linux/amd64`

### Description

- **What** is the problem: Server crashed with an index out of range error, dumping a fairly concise trace.
- **Who** is affected: Users of my instance, including myself
- **How** is this bug manifesting: [dendrite_crash2.log](https://github.com/matrix-org/dendrite/files/12796732/dendrite_crash2.log)
- **When** did this first appear: This is the first time I've seen this specific crash, the server was idle when it occurred.

### Steps to reproduce

- Start a server with federation enabled
- Join a handful of populated rooms
- Leave it alone for a while (approx. 2 days)

Start by examining the linked dendrite_crash2.log for the full Go stack trace, using the reported Dendrite commit and PostgreSQL 15.4 environment as context. Trace the failing path from the stack location and investigate the idle federated-server scenario; done means the reported out-of-range crash is prevented and the scenario has regression coverage.

Written by the indexing model from the issue text.