FIX: UPGRADE_UNIT not allowed on converting units

Prevents exploits/bugs where upgraded unit type would inherit ACTIVITY_CONVERT that may be illegal for its type.

freeciv/freeciv/common/unit.c

@@ -2063,6 +2063,15 @@ enum unit_upgrade_result unit_upgrade_test(const struct unit *punit,
   struct city *pcity;
   int cost;
 
+  if (punit->activity == ACTIVITY_CONVERT) {
+    /*  TODO: There may be other activities that the upgraded unit is not
+       allowed to do, which we could also test.
+       -
+       If convert were legal for new unit_type we could allow, but then it 
+       has a 'head start' getting activity time from the old conversion. */
+    return UU_NOT_ACTIVITY;
+  }
+
   if (!to_unittype) {
     return UU_NO_UNITTYPE;
   }
@@ -2196,6 +2205,12 @@ enum unit_upgrade_result unit_upgrade_info(const struct unit *punit,
                 utype_name_translation(to_unittype),
                 unit_name_translation(punit->transporter));
     break;
+  case UU_NOT_ACTIVITY:
+    fc_snprintf(buf, bufsz,
+                _("Cannot upgrade %s while doing '%s'."),
+                utype_name_translation(from_unittype),
+                unit_activity_name(punit->activity));
+    break;
   }
 
   return result;

freeciv/freeciv/common/unit.h

@@ -63,8 +63,9 @@ enum unit_upgrade_result {
   UU_NOT_IN_CITY,
   UU_NOT_CITY_OWNER,
   UU_NOT_ENOUGH_ROOM,
-  UU_NOT_TERRAIN,         /* The upgraded unit could not survive. */
-  UU_UNSUITABLE_TRANSPORT /* Can't upgrade inside current transport. */
+  UU_NOT_TERRAIN,          /* The upgraded unit could not survive. */
+  UU_UNSUITABLE_TRANSPORT, /* Can't upgrade inside current transport. */
+  UU_NOT_ACTIVITY          /* Can't upgrade during this activity (e.g., CONVERT) */
 };
 
 enum unit_airlift_result {