[std][BTree] Fix behavior of ::append to match documentation and ::insert

Author: tinnamchoi

library/alloc/src/collections/btree/append.rs

@@ -107,6 +107,11 @@ where
     /// If two keys are equal, returns the key-value pair from the right source.
     fn next(&mut self) -> Option<(K, V)> {
         let (a_next, b_next) = self.0.nexts(|a: &(K, V), b: &(K, V)| K::cmp(&a.0, &b.0));
-        b_next.or(a_next)
+        match (a_next, b_next) {
+            (Some((a_k, _)), Some((_, b_v))) => Some((a_k, b_v)),
+            (Some(a), None) => Some(a),
+            (None, Some(b)) => Some(b),
+            (None, None) => None,
+        }
     }
 }