message: cheap_heap: ensure we don't silently overflow arena
diff: @@ -45,7 +45,9 @@ public:
  }
  const auto off = _arenaOff++;
  - return ptrFromOffset(off);
  + const auto ptr = ptrFromOffset(off);
  + hard_assert(ptr < arenaEnd());
  + return ptr;
  }
  constexpr size_t getSize(void *ptr) const {
  @@ -119,7 +121,9 @@ public:
  }
  const auto off = _arenaOff++;
  - return ptrFromOffset(off);
  + const auto ptr = ptrFromOffset(off);
  + hard_assert(ptr < arenaEnd());
  + return ptr;
  }
  size_t getSize(void *ptr) const {
  