Fix a crash for governors without specialists

Governors that forbid specialists do not bother computing how much
specialists would produce and end up with an empty specialist output
cache. When applying solutions to evaluate them, however, we were trying
to use the cached values. Only do it when there specialists are actually
present in the solution.

See longturn#2332.

common/city.cpp

@@ -2244,14 +2244,19 @@ void add_specialist_output(
   {
     int count = pcity->specialists[sp];
 
-    output_type_iterate(stat_index)
-    {
-      int amount = pcsoutputs ? pcsoutputs->at(sp)[stat_index]
-                              : get_specialist_output(pcity, sp, stat_index);
+    // This is more than just an optimization. For governors that forbid
+    // specialists, the cache may not be filled.
+    if (count > 0) {
+      output_type_iterate(stat_index)
+      {
+        int amount = pcsoutputs
+                         ? pcsoutputs->at(sp)[stat_index]
+                         : get_specialist_output(pcity, sp, stat_index);
 
-      output[stat_index] += count * amount;
+        output[stat_index] += count * amount;
+      }
+      output_type_iterate_end;
     }
-    output_type_iterate_end;
   }
   specialist_type_iterate_end;
 }