Fixes#348, reported by Jeroen Meijer.
* bin/autfilt.cc: If -B is used with many --product,
degeneralize intermediate products as needed.
* NEWS: Mention the change.
* tests/core/prodchain.test: New file.
* tests/Makefile.am: Add it.
* spot/twa/acc.cc, spot/twa/acc.hh: Fix reporting of
overflow.
* tests/core/acc.cc: Adjust.
