message: add communicator size check to copy/move assignment
diff: @@ -357,6 +357,8 @@ Matrix<ValueType, LocalIndexType, GlobalIndexType>::operator=(
  const Matrix& other)
  {
  if (this != &other) {
  + GKO_ASSERT_EQ(other.get_communicator().size(),
  + this->get_communicator().size());
  this->set_size(other.get_size());
  diag_mtx_->copy_from(other.diag_mtx_.get());
  offdiag_mtx_->copy_from(other.offdiag_mtx_.get());
  @@ -380,6 +382,8 @@ Matrix<ValueType, LocalIndexType, GlobalIndexType>::operator=(
  Matrix&& other) noexcept
  {
  if (this != &other) {
  + GKO_ASSERT_EQ(other.get_communicator().size(),
  + this->get_communicator().size());
  this->set_size(other.get_size());
  other.set_size({});
  diag_mtx_->move_from(other.diag_mtx_.get());
  