Fix a very old bug in mod2_preinv.
Fix a very old bug in mod2_preinv.

wbhart/flint2,jpflori/flint2,fredrik-johansson/flint2,dsroche/flint2,jpflori/flint2,dsroche/flint2,wbhart/flint2,jpflori/flint2,dsroche/flint2,jpflori/flint2,dsroche/flint2,wbhart/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2