mtc_api: Make LandmarkSequence::add more robust

If the number of landmarks is greater than the max, then drain the
oldest landmarks from the deque. This makes the code more robust in case
we ever decode a `LandmarkSequence` with a different `max_landmarks`
than when it was encoded.

Author: cjpatton

crates/mtc_api/src/landmark.rs

@@ -50,10 +50,9 @@ impl LandmarkSequence {
                 ));
             }
         }
-        // Keep `max_landmarks + 1` tree sizes, since we want `max_landmarks`
-        // landmark intervals.
-        if self.landmarks.len() == self.max_landmarks + 1 {
-            self.landmarks.pop_front();
+        if self.landmarks.len() >= self.max_landmarks {
+            self.landmarks
+                .drain(..self.landmarks.len() - self.max_landmarks);
         }
         self.landmarks.push_back(tree_size);
         self.last_landmark += 1;