While coalescing, add new block to the free list if current + next is fine

src/freelist/coalesce.c

@@ -99,6 +99,7 @@ void coalesce(freelist_rb_t *raw_block, block_header_t *ptr) {
             raw_block->border_ptr = ptr;
         }
         set_size_and_free(raw_block, ptr, current_next_size);
+        add_block(raw_block, ptr);
         return;
     }