matrix-free FEEvaluation: initialize with nan

Initialize the internal scratch_data of FEEvaluation with signaling_nans
in debug mode.

include/deal.II/matrix_free/fe_evaluation_data.h

@@ -29,6 +29,7 @@
 #include <deal.II/base/template_constraints.h>
 #include <deal.II/base/tensor.h>
 #include <deal.II/base/vectorization.h>
+#include <deal.II/base/signaling_nan.h>
 
 #include <deal.II/matrix_free/dof_info.h>
 #include <deal.II/matrix_free/mapping_info_storage.h>
@@ -1183,7 +1184,13 @@ FEEvaluationData<dim, Number, is_face>::set_data_pointers(
      n_quadrature_points);
 
   const unsigned int allocated_size = size_scratch_data + size_data_arrays;
+#  ifdef DEBUG
+  scratch_data_array->clear();
+  scratch_data_array->resize(allocated_size,
+                             Number(numbers::signaling_nan<double>()));
+#  else
   scratch_data_array->resize_fast(allocated_size);
+#  endif
   scratch_data.reinit(scratch_data_array->begin() + size_data_arrays,
                       size_scratch_data);