Fix CHOLMOD leak

Oliver Sanderrequested to merge
fix-cholmod-leak-in-2.10 into releases/2.10

When calling CHOLMOD to compute the Cholesky factor of a matrix, it was not tested whether such a factor already existed. As a consequence, if the solver was called twice in a row, the old factor would be lost to a memory leak.

Fix this by explicitly deallocating the old Cholesky factor before asking CHOLMOD for a new one.

(cherry picked from commit e4ffbb1f)

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