AP_Mission: Allow Param4 to be uploaded with NaN

By default, QGroundControl will attempt to upload Loiter Unlimited with a NaN in param4.
Given this field could be NaN, we allow it through the parser.

See: https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_LOITER_UNLIM

libraries/AP_Mission/AP_Mission.cpp

@@ -964,6 +964,9 @@ MAV_MISSION_RESULT AP_Mission::sanity_check_params(const mavlink_mission_item_in
     case MAV_CMD_NAV_WAYPOINT:
         nan_mask = ~(1 << 3); // param 4 can be nan
         break;
+    case MAV_CMD_NAV_LOITER_UNLIM:
+        nan_mask = ~(1 << 3); // param 4 can be nan
+        break;
     case MAV_CMD_NAV_LAND:
         nan_mask = ~(1 << 3); // param 4 can be nan
         break;