Fix 2095

[fwesselm]

Fix #2095:

• Pointers stored in HighsMatrixSlice may get invalidated if the coefficient matrix is reallocated (e.g. when non-zeros are added in HPresolve::addToMatrix).
• The proposed fix is to directly iterate over the non-zero elements (positions in the matrix) instead of using HighsMatrixSlice when the matrix is modified while iterating.

[jajhall]

Looks correct, and more robust than my fix. I guess you'll regression test it

Are there other places in presolve where such an issue could be exposed?

[fwesselm]

Looks correct, and more robust than my fix. I guess you'll regression test it

Are there other places in presolve where such an issue could be exposed?

Regression testing is ongoing.

I looked for calls to HPresolve::addToMatrix while iterating over a HighsMatrixSlice in presolve, and I think the two occurrences in this PR are the only problematic ones. I hope I did not overlook something.

[fwesselm]

Testing shows that, while HiGHS behavior is affected, the overall results look reasonable.