Make sure aligned_alloc args are valid

src/arena_allocator.hpp

@@ -93,7 +93,8 @@ private:
      * @throws std::bad_alloc if memory allocation fails
      */
     static Block *create(size_t size, Block *prev) {
-      void *memory = std::aligned_alloc(alignof(Block), sizeof(Block) + size);
+      void *memory = std::aligned_alloc(
+          alignof(Block), align_up(sizeof(Block) + size, alignof(Block)));
       if (!memory) {
         throw std::bad_alloc();
       }