Adds safety checks to scripting orders with waypoints (#5173)

* Adds safety checks to scripting orders with waypoints

Turns out setting a waypoint order then trying to get to target of that order in the same frame triggers a crash in the scripting API for ship orders (`order.cpp` line 295). This was due to invalid lookups which lacked safety checks.

This PR adds those safety checks. Now, if this situation is encountered, it leaves `objnum` at -1 and thus returns an object for which `isValid` is false.

Fix is tested and works as expected. Also fixes #5172

* appease clang

* cleanup thanks to Goober

* clang again

* forgot to re-add the nullptr

* remove redundant checks

Author: wookieejedi

code/scripting/api/objs/order.cpp

@@ -289,11 +289,15 @@ ADE_VIRTVAR(Target, l_Order, "object", "Target of the order. Value may also be a
 			break;
 		case AI_GOAL_WAYPOINTS:
 		case AI_GOAL_WAYPOINTS_ONCE:
-			wpl = find_matching_waypoint_list(ohp->aigp->target_name);
-			if(ohp->odx == 0) {
+			// check if waypoint order is the current goal (ohp->odx == 0) and if it is valid
+			if ( (ohp->odx == 0) && (aip->wp_index != INVALID_WAYPOINT_POSITION) &&
+				(aip->wp_index < aip->wp_list->get_waypoints().size()) ) {
 				objnum = aip->wp_list->get_waypoints()[aip->wp_index].get_objnum();
 			} else {
-				objnum = wpl->get_waypoints().front().get_objnum();
+				wpl = find_matching_waypoint_list(ohp->aigp->target_name);
+				if (wpl != nullptr) {
+					objnum = wpl->get_waypoints().front().get_objnum();
+				}
 			}
 			break;
 		case AI_GOAL_STAY_STILL: