Adding code to help Valgrind find undefined behaviour in clause pool
Allow to check for unreachable code
Better indentation of code
Don't use a freed clause's data

BVA used a freed clause's data. This is not good practice, even though
we have a pool and the data is still there and untouched.

The new system might actually be faster, too, as well as more correct
Don't use -Wunreachable-code

It conflicts with defensive programming