Fix invalid memory write in k-order DLL

The bug would show at order 3 when the last output argument (derivs) is not
requested (in practice every 3rd order solution without pruning). The DLL would
still attempt to write into it, causing an invalid memory access.

mex/sources/k_order_perturbation/k_order_perturbation.cc

@@ -293,7 +293,7 @@ extern "C" {
                 ++ii;
 
               }
-            if (kOrder == 3 && nlhs > 4)
+            if (kOrder == 3 && nlhs > 5)
               {
                 const FGSContainer *derivs = app.get_rule_ders();
                 const std::string fieldnames[] = {"gy", "gu", "gyy", "gyu", "guu", "gss",