Fix a class of crashes in the city list

Modifying a city can (somehow) change the selection, upon which selected_cities
is modified. Modifying a QList invalidates iterators, thus we were triggering
undefined behavior when doing this in loops. Copy the list before doing
anything with it, which avoids any invalidation (since we work on the copy).

client/cityrep.cpp

@@ -424,7 +424,9 @@ void city_widget::clear_worlist()
   struct worklist empty;
   worklist_init(&empty);
 
-  for (auto *pcity : qAsConst(selected_cities)) {
+  const auto saved_selection =
+      selected_cities; // Copy to avoid invalidations
+  for (auto *pcity : saved_selection) {
     Q_ASSERT(pcity != nullptr);
     city_set_worklist(pcity, &empty);
   }
@@ -435,7 +437,9 @@ void city_widget::clear_worlist()
  */
 void city_widget::buy()
 {
-  for (auto *pcity : qAsConst(selected_cities)) {
+  const auto saved_selection =
+      selected_cities; // Copy to avoid invalidations
+  for (auto *pcity : saved_selection) {
     Q_ASSERT(pcity != nullptr);
     cityrep_buy(pcity);
   }
@@ -642,7 +646,9 @@ void city_widget::display_list_menu(const QPoint)
     }
     city_list_iterate_end;
 
-    for (auto *pcity : qAsConst(selected_cities)) {
+    const auto saved_selection =
+        selected_cities; // Copy to avoid invalidations
+    for (auto *pcity : saved_selection) {
       if (nullptr != pcity) {
         switch (m_state) {
         case CHANGE_PROD_NOW:
@@ -687,13 +693,10 @@ void city_widget::display_list_menu(const QPoint)
           }
           break;
         case CMA:
-          if (nullptr != pcity) {
-            if (CMA_NONE == id) {
-              cma_release_city(pcity);
-            } else {
-              cma_put_city_under_agent(pcity,
-                                       cmafec_preset_get_parameter(id));
-            }
+          if (CMA_NONE == id) {
+            cma_release_city(pcity);
+          } else {
+            cma_put_city_under_agent(pcity, cmafec_preset_get_parameter(id));
           }
 
           break;