message: ubsan: Fix type mismatch for xml_extra_buffer in compact mode
  We were using allocate_memory to allocate struct xml_extra_buffer that
  contains pointers; with compact mode, this allocation can be misaligned
  by 4b with 8b pointers; fix this by manually realigning the pointer.
diff: @@ -6076,11 +6076,17 @@ namespace pugi
  // get extra buffer element (we'll store the document fragment buffer there so that we can deallocate it later)
  impl::xml_memory_page* page = 0;
  - impl::xml_extra_buffer* extra = static_cast<impl::xml_extra_buffer*>(doc->allocate_memory(sizeof(impl::xml_extra_buffer), page));
  + impl::xml_extra_buffer* extra = static_cast<impl::xml_extra_buffer*>(doc->allocate_memory(sizeof(impl::xml_extra_buffer) + sizeof(void*), page));
  (void)page;
  if (!extra) return impl::make_parse_result(status_out_of_memory);
  + #ifdef PUGIXML_COMPACT
  + // align the memory block to a pointer boundary; this is required for compact mode where memory allocations are only 4b aligned
  + // note that this requires up to sizeof(void*)-1 additional memory, which the allocation above takes into account
  + extra = reinterpret_cast<impl::xml_extra_buffer*>((reinterpret_cast<uintptr_t>(extra) + (sizeof(void*) - 1)) & ~(sizeof(void*) - 1));
  + #endif
  +
  // add extra buffer to the list
  extra->buffer = 0;
  extra->next = doc->extra_buffers;
  