Fix bugs in heap-delete

Fix bugs in heap-delete

When an entry is kept to be use later with `heap-delete`, its `index`
in the owning heap is automatically updated as the entry sifts up
or down.

However, if the entry is removed from the heap via either a `heap-pop`
or a `heap-delete` operation, its index is not invalidated and
the entry can still be used later with `heap-delete` and remove the
wrong element from the heap.

This patch invalidates entries when they leave the heap by setting
their index to `f`, and check the index in `entry>index`.