Check index rather than item for short-circuit

Author: ejmount

src/k_smallest.rs

@@ -124,12 +124,15 @@ where
         }
 
         let (left_idx, right_idx) = children_of(origin);
-        let (left_item, right_item) = (self.get(left_idx), self.get(right_idx));
 
-        if left_item.is_none() { // the left is the earlier child, so if it doesn't exist there's nothing to swap with
+        if left_idx >= self.len {
+            // the left is the earlier child, so if it doesn't exist there's nothing to swap with
             return;
         }
 
+        let (left_item, right_item) = (self.get(left_idx), self.get(right_idx));
+
+
         let cmp = self
             .compare(left_item, right_item)
             .unwrap_or(Ordering::Greater); // The right item may not exist, so default to picking the left, i.e. lower