Fix obj constant on max. Fix undefined memory access at root

[chris-maes]

This PR fixes two bugs discovered while testing the AMPL interface:

1. We weren't correctly calculating the objective constant when presolve eliminated variables on a maximization problem.

Thanks to Alice for the quick fix!

2. We had an undefined memory access on the number of nodes and simplex iterations when solving a MIP at the root node.

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[chris-maes]

/ok to test

[copy-pr-bot]

/ok to test

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[rgsl888prabhu]

/ok to test 7565dd7

[chris-maes]

/ok to test c6d9f6d

[chris-maes]

/ok to test b0e612e

[chris-maes]

/merge

[chris-maes]

/merge