[Kernels] Only clear out target vector once in ELL mv kernel

dune/common/kernel/ell/mv.hh

@@ -51,10 +51,14 @@ namespace Dune {
           for (size_type block = 0; block < n; ++block)
             {
               size_type cols = (mat_block_offset[block+1] - mat_block_offset[block]) >> Memory::block_size_log2<kernel_block_size>::value;
+
+              // zero out existing values
+              for (int i = 0; i < kernel_block_size; ++i)
+                y[block*kernel_block_size + i] = 0;
+
+              // accumulate matrix-vector product
               for (int j = 0; j < cols; ++j)
                 {
-                  for (int i = 0; i < kernel_block_size; ++i)
-                    y[block*kernel_block_size + i] = 0;
                   for (int i = 0; i < kernel_block_size; ++i)
                     y[block*kernel_block_size + i] += mat_data[offset*kernel_block_size + kernel_block_size*j+i] * x[mat_col[offset*kernel_block_size + kernel_block_size*j+i]];
                 }