Index the session past correctly when discarding.

servo · bors-servo · Jan 6, 2017 · Jan 5, 2017 · Jan 5, 2017 · Jan 5, 2017
commit 5fd284cf3ac6c874d44abaa7983726fd4fb4e158
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
@@ -2110,7 +2110,8 @@ 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 {