linked_list_allocator vulnerable to out-of-bound writes on `Heap` initialization and `Heap::extend`

Summary

linked_list_allocator is an allocator usable for no_std systems. Prior to version 0.10.2, the heap initialization methods were missing a minimum size check for the given heap size argument. This could lead to out-of-bound writes when a heap was initialized with a size smaller than `3 * size_of::<usize>` because of metadata write operations. This vulnerability impacts all the initialization functions on the `Heap` and `LockedHeap` types, including `Heap::new`, `Heap::init`, `Heap::init_from_slice`, and `LockedHeap::new`. It also affects multiple uses of the `Heap::extend` method. Version 0.10.2 contains a patch for the issue. As a workaround, ensure that the heap is only initialized with a size larger than `3 * size_of::<usize>` and that the `Heap::extend` method is only called with sizes larger than `2 * size_of::<usize>()`. Also, ensure that the total heap size is (and stays) a multiple of `2 * size_of::<usize>()`.