Auto merge of #14860 - asajeffrey:constellation-index-past-correctly-when-discarding, r=cbrewster

Index the session past correctly when discarding.

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Oops, indexed from the wrong end when discarding documents in #14312.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we're not testing document discarding

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components/constellation/constellation.rs

@@ -2102,7 +2102,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         } else if let Some(frame) = self.frames.get_mut(&frame_change.frame_id) {
             debug!("Adding pipeline to existing frame.");
             frame.load(frame_change.new_pipeline_id, frame_change.url.clone());
-            let evicted_id = frame.prev.get_mut(opts::get().max_session_history)
+            let evicted_id = frame.prev.len().checked_sub(opts::get().max_session_history)
+                .and_then(|index| frame.prev.get_mut(index))
                 .and_then(|entry| entry.pipeline_id.take());
             (evicted_id, false, true, true)
         } else {