Fallback to legacy behaviour if empty caches are detected

Fallback to the behavior used prior to the introduction of these
caches, if the assertion checking for an empty cache is triggered.

While here, move an assertion outside a loop to make sure it is not
checked for each iteration.

common/city.cpp

@@ -2248,7 +2248,8 @@ void add_specialist_output(
     // specialists, the cache may not be filled.
     if (count > 0) {
       // If there is a cache it must not be empty.
-      fc_assert(!pcsoutputs || !pcsoutputs->empty());
+      fc_assert_action(!pcsoutputs || !pcsoutputs->empty(),
+                       pcsoutputs = nullptr);
 
       output_type_iterate(stat_index)
       {
@@ -2873,15 +2874,15 @@ void set_city_production(struct city *pcity,
   trade_routes_iterate_end;
   pcity->prod[O_GOLD] += get_city_tithes_bonus(pcity);
 
+  // If there is a cache it must not be empty.
+  fc_assert_action(!pcwaste || !pcwaste->empty(), pcwaste = nullptr);
+
   /* Account for waste.  Note that waste is calculated before tax income is
    * calculated, so if you had "science waste" it would not include taxed
    * science.  However waste is calculated after the bonuses are multiplied
    * on, so shield waste will include shield bonuses. */
   output_type_iterate(o)
   {
-    // If there is a cache it must not be empty.
-    fc_assert(!pcwaste || !pcwaste->empty());
-
     pcity->waste[o] =
         city_waste(pcity, o, pcity->prod[o] * pcity->bonus[o] / 100, nullptr,
                    gov_centers, pcwaste ? &pcwaste->at(o) : nullptr);