message: meshable-arena: fix alignment check
diff: @@ -170,7 +170,7 @@ Span MeshableArena::reservePages(Length pageCount, Length pageAlignment) {
  d_assert(!result.empty());
  - if (unlikely(pageAlignment > 1 && result.offset % pageAlignment != 0)) {
  + if (unlikely(pageAlignment > 1 && ((ptrvalFromOffset(result.offset) / kPageSize) % pageAlignment != 0))) {
  freeSpan(result);
  // recurse once, asking for enough extra space that we are sure to
  // be able to find an aligned offset of pageCount pages within.
  @@ -248,6 +248,8 @@ void *MeshableArena::pageAlloc(size_t pageCount, void *owner, size_t pageAlignme
  d_assert(pageCount < std::numeric_limits<Length>::max());
  auto span = reservePages(pageCount, pageAlignment);
  + d_assert((reinterpret_cast<uintptr_t>(ptrFromOffset(span.offset)) / kPageSize) % pageAlignment == 0);
  +
  const auto off = span.offset;
  d_assert(ptrFromOffset(span.offset) < arenaEnd());
  