message: fix wrong hint in map insertion in slab alloc
diff: @@ -574,6 +574,7 @@ void SlabAlloc::do_free(ref_type ref, char* addr)
  REALM_ASSERT_RELEASE_EX(ref + size <= next->first, ref, size, next->first, next->second, get_file_path_for_assertions());
  // See if element can be combined with next element
  if (ref + size == next->first) {
  + // if so, combine to include next element and remove that from collection
  size += next->second;
  next = m_free_read_only.erase(next);
  }
  @@ -590,7 +591,7 @@ void SlabAlloc::do_free(ref_type ref, char* addr)
  prev->second += size;
  return; // Done!
  }
  - m_free_read_only.emplace_hint(prev, ref, size); // Throws
  + m_free_read_only.emplace_hint(next, ref, size); // Throws
  }
  else {
  m_free_read_only.emplace(ref, size); // Throws
  