Fix CHOLMOD leak

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)

enabled an automatic merge when all merge checks for 1368ef43 pass

mentioned in commit 21c67275

merged