message: Fixing wrong index for null block test
diff: @@ -155,7 +155,7 @@ const double& BlockMatrix::Elem (int i, int j) const
  findGlobalRow(i, iblock, iloc);
  findGlobalCol(j, jblock, jloc);
  - if (IsZeroBlock(i, j))
  + if (IsZeroBlock(iblock, jblock))
  {
  mfem_error("BlockMatrix::Elem");
  }
  