forgot to add mem::forget() on key

Author: asder8215

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

@@ -3488,9 +3488,10 @@ impl<'a, K: Ord, V, A: Allocator + Clone> CursorMutKey<'a, K, V, A> {
 
         let mut emptied_internal_root = false;
         if let Ok(next_kv) = current.next_kv() {
-            let ((_, val), pos) =
+            let ((key, val), pos) =
                 next_kv.remove_kv_tracking(|| emptied_internal_root = true, self.alloc.clone());
             // Don't run destructor on next element's value
+            mem::forget(key);
             mem::forget(val);
             self.current = Some(pos);
             *self.length -= 1;