message: improve readability of update_reader_view()
diff: @@ -1127,9 +1127,10 @@ void SlabAlloc::update_reader_view(size_t file_size)
  // 0. Special case: figure out if extension is to be done entirely within a single
  // existing mapping. This is the case if the new baseline (which must be larger
  // then the old baseline) is still below the old base of the slab area.
  + if (file_size < old_slab_base) {
  + REALM_ASSERT(old_num_mappings > 0);
  const auto earlier_last_index = old_num_mappings - 1;
  MapEntry& cur_entry = m_mappings[earlier_last_index];
  - if (file_size < old_slab_base) {
  const size_t section_start_offset = get_section_base(earlier_last_index);
  const size_t section_size = file_size - section_start_offset;
  requires_new_translation = true;
  @@ -1146,6 +1147,9 @@ void SlabAlloc::update_reader_view(size_t file_size)
  // 1. figure out if there is a partially completed mapping, that we need to extend
  // to cover a full mapping section
  if (old_baseline < old_slab_base) {
  + REALM_ASSERT(old_num_mappings > 0);
  + const auto earlier_last_index = old_num_mappings - 1;
  + MapEntry& cur_entry = m_mappings[earlier_last_index];
  const size_t section_start_offset = get_section_base(earlier_last_index);
  const size_t section_size = old_slab_base - section_start_offset;
  // we could not extend the old mapping, so replace it with a full, new one
  