Investigate correctness of MapEmbeddedItemsChunk

[stefan-baumann]

The parsing of this chunk behaves unexpectedly on some maps, e.g. on https://tm.mania-exchange.com/maps/12048, consuming 12 bytes less than expected.
As far as I can tell looking at it on the surface, it only fails on maps which already don't contain any embedded items, so there are no relevant practical implications, but this should still be investigated in the future.