message: freelist: add assertion
diff: @@ -149,6 +149,7 @@ public:
  _end = _start + mh->spanSize();
  const auto allocCount = init(prng, mh->writableBitmap());
  + d_assert(allocCount > 0);
  // tell the miniheap how many offsets we pulled out/preallocated into our freelist
  mh->incrementInUseCount(allocCount);
  