CGStep: resize `p_` when size of problem might change

dune/solvers/iterationsteps/cgstep.hh

@@ -43,6 +43,7 @@ namespace Dune {
                 this->setMatrix(mat);
                 setProblem(x);
                 r_ = rhs;
+                p_.resize(r_.size());
             }
 
             void check() const override;