There is a rather large design flaw in FreeSpace debriefings... debriefing SEXPs can refer to in-mission ship data, while all ships (and all objects) are deleted as soon as the mission completes. Specifically, `obj_delete()` is called which sets all objects to `OBJ_NONE` and marks them as freed, although the data still exists in the various arrays. This is the use-after-free problem, but applied to the in-game object system, rather than memory accessed by pointers.
It's probably best to defer fixes to an as-needed basis. As for hitpoint SEXPs, this change updates them to use the object and ship references from the ship registry, since the ship registry is not cleared by the time the debriefing is loaded. (Using `objp->instance` and `shipp->objnum` at this point would be problematic.) Since subsystems are also cleared at this point, this also updates the warning messages in those functions.
Fixes an Assert seen in debug mode in Inferno.
Goober5000