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.

dune/istl/cholmod.hh

@@ -461,6 +461,10 @@ public:
 
     });
 
+    // 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_);