* 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
* add subsystem indexer and ensure that ship_get_indexed_subsys and ship_get_first_subsys are independent
* also add reverse lookup from subsys to index
* update indexer to accommodate subsystem pointers being changed
* use clearer function names, and fix the type of subsys_list_indexer
* add assertions
* address latest feedbcak
This adds a new AI goal which is equivalent to play-dead except that it "sticks". It doesn't clear the goal list
so it will continue playing dead until the goal is removed or another goal with a higher priority is added.
wookieejedi