cosmetics: unify indentation
MemReadIOCallback, SafeReadIOCallback: add new classes for safe (memory) reading

squash! EbmlMemoryStream: add new class for safe memory reading
EbmlMaster::Read(): drop elements with unset values in SCOPE_FULL_DATA mode

This prevents elements that couldn't be read properly (e.g. due to
defective data) from being handed over to the calling code in at best
half initialized state. See https://trac.bunkus.org/ticket/1096