use atomic read/write on the page->heap field where concurrent interaction is possible

src/alloc.c

@@ -235,7 +235,7 @@ static mi_decl_noinline void _mi_free_block_mt(mi_page_t* page, mi_block_t* bloc
   }
   else {
     // racy read on `heap`, but ok because MI_DELAYED_FREEING is set (see `mi_heap_delete` and `mi_heap_collect_abandon`)
-    mi_heap_t* heap = page->heap;
+    mi_heap_t* heap = (mi_heap_t*)mi_atomic_read_ptr(mi_atomic_cast(void*, &page->heap));
     mi_assert_internal(heap != NULL);
     if (heap != NULL) {
       // add to the delayed free list of this heap. (do this atomically as the lock only protects heap memory validity)