fix: Add range check to signature index

[SamHSmith]

Context

The bug is triggered if a a peer is unregistered from the topology while Sumeragi Messages are still in flight. This is the only place where we use and array index operation that can panic. It is very hard to write a test for this case that is guaranteed to show the issue each time.

Closes #5104

Solution

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Checklist

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[mversic]

what's the corresponding issue for this?

[0x009922]

Linked it: #5104