Fix hang during output of (7,6).

Fix hang during output of (7,6).

I had rather hoped that GLPK would always output a min fragment
slightly _less_ than the true rational value, on the basis that some
of its minima would be slightly below the right value and some
slightly above. Sadly, (7,6) consistently guesses high, so I must fall
back to the usual 1e10 bodge to round to rationals.