Initialize uninitialized postprocess_sides flag

This was broken ten years ago and I never caught it until an -fsanitize
run.  We need to get the full -fsanitize, along with whatever -W option
would have caught this, into CI ASAP.

Author: roystgnr

src/systems/diff_system.C

@@ -39,6 +39,7 @@ DifferentiableSystem::DifferentiableSystem(EquationSystems & es,
   Parent      (es, name_in, number_in),
   time_solver (),
   deltat(1.),
+  postprocess_sides(false),
   print_solution_norms(false),
   print_solutions(false),
   print_residual_norms(false),