debmd: Fix DebMd parsing of Packages files

Debian packages file use a newline to signal the end of a record, not
Size. While e.g. the main Debian archive always uses Size as the last
record that's not the case for e.g. the Raspbian archives.
Merge pull request #814 from sjoerd-ccu/fix-debmd-parsing

debmd: Fix DebMd parsing of Packages files