Zero-initialize structure to avoid returning undefined

The compiler will otherwise complain that we are returning undefined
data. There is no way for the client code to know whether this was
the case.

opm/core/linalg/LinearSolverUmfpack.cpp

@@ -56,7 +56,7 @@ namespace Opm
             const_cast<double*>(sa)
         };
         call_UMFPACK(&A, rhs, solution);
-        LinearSolverReport rep;
+        LinearSolverReport rep = {0};
         rep.converged = true;
         return rep;
     }