set extra debug padding per-heap

include/mimalloc-types.h

@@ -334,6 +334,7 @@ struct mi_heap_s {
   size_t                page_count;                          // total number of pages in the `pages` queues.
   size_t                page_retired_min;                    // smallest retired index (retired pages are fully free, but still in the page queues)
   size_t                page_retired_max;                    // largest retired index into the `pages` array.
+  size_t                extra_padding;                       // extra padding bytes in each heap block to better detect heap block overflows
   mi_heap_t*            next;                                // list of heaps per thread
   bool                  no_reclaim;                          // `true` if this heap should not reclaim abandoned pages
 };