Fix memory leak in the CHOLMOD wrapper code

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.

I am amazed that this has gone unnoticed for so long. It's a massive leak.

The fix should go into 2.10 as well.

dune/istl/cholmod.hh

@@ -461,10 +461,11 @@ public:
 
     });
 
-    // Now analyse the pattern and optimal row order
+    // Remove old factor that may be left over from a previous run
     if (L_)
       CholmodMethod::free_factor(&L_, &c_);
 
+    // Now analyse the pattern and optimal row order
     L_ = CholmodMethod::analyze(M.get(), &c_);
 
     // Do the factorization (this may take some time)