Handle empty specialists output cache gracefully

Discovered while debugging the following behavior: Disallowing
specialists for a governor that already has assigned specialists
crashed the client.

The cause was that in `add_specialist_output` accessing items in
`pcsoutputs` failed, as it was empty.

Closes #2331.

common/city.cpp

@@ -2245,8 +2245,9 @@ void add_specialist_output(
 
     output_type_iterate(stat_index)
     {
-      int amount = pcsoutputs ? pcsoutputs->at(sp)[stat_index]
-                              : get_specialist_output(pcity, sp, stat_index);
+      int amount = pcsoutputs && !pcsoutputs->empty()
+                       ? pcsoutputs->at(sp)[stat_index]
+                       : get_specialist_output(pcity, sp, stat_index);
 
       output[stat_index] += count * amount;
     }