AP_Scripting: plane_ship_landing.lua: add pos/vel sanity check

this method can return false if the target is outside the AP_Follow acceptance radius.  So we can be receiving messages from the target, so have_target can be true, but false is returned from get_target_location_and_velocity_ofs, meaning we try to call methods on nil values

Author: peterbarker

libraries/AP_Scripting/applets/plane_ship_landing.lua

@@ -337,13 +337,23 @@ function update_target()
       have_target = false
       return
    end
+
+   target_pos, target_velocity = follow:get_target_location_and_velocity_ofs()
+   if target_pos == nil or target_velocity == nil then
+      if have_target then
+         gcs:send_text(MAV_SEVERITY.WARNING, "No target location/velocity")
+         have_target = false
+      end
+      return
+   end
+
    if not have_target then
       gcs:send_text(MAV_SEVERITY.INFO, "Have beacon")
       arming:set_aux_auth_passed(auth_id)
    end
+
    have_target = true
 
-   target_pos, target_velocity = follow:get_target_location_and_velocity_ofs()
    target_pos:change_alt_frame(ALT_FRAME_ABSOLUTE)
    target_heading = follow:get_target_heading_deg()
    -- zero vertical velocity to reduce impact of ship movement