bugfix in operator*

[[Imported from SVN: r1494]]

dune/istl/matrix.hh

@@ -320,7 +320,7 @@ namespace Dune {
     /// Generic matrix multiplication.
     friend Matrix<T> operator*(const Matrix<T>& m1, const Matrix<T>& m2) {
       Matrix<T> out(m1.N(), m2.M());
-      out.clear();
+      out = 0;
 
       for (size_type i=0; i<out.N(); i++ ) {
         for ( size_type j=0; j<out.M(); j++ )