Also initialize preconditioner_ in default constructor

preconditioner_ needs to be initialized in the default
constructor. Otherwise, when the default constructor and later
setProblem are used, its value is undefined.

dune/solvers/iterationsteps/cgstep.hh

@@ -13,6 +13,7 @@ namespace Dune {
         {
         public:
             CGStep()
+                : preconditioner_(nullptr)
             {}
 
             CGStep(const MatrixType& matrix,