apt.dat parser: better input stream handling

apt.dat parser: better input stream handling

- use ( in.getline(...) ) as the main loop condition instead of
  ( ! in.eof() ). This should behave better (see
  <https://gehrcke.de/2011/06/reading-files-in-c-using-ifstream-dealing-correctly-with-badbit-failbit-eofbit-and-perror/>);
- check in.bad() after exiting from each reading loop; if the exit was
  caused by an error, log an appropriate message and throw an
  exception.