clear ship registry after mission

Create a `ship_level_close()` function and call it from `game_level_close()`.  Among other tasks, this clears the ship registry to prevent SEXPs or scripts from obtaining references to ships that have been deleted.

Follow-up to #7522.

Author: Goober5000

code/ship/ship.cpp

@@ -6914,6 +6914,18 @@ void ship_level_init()
 	Man_thruster_reset_timestamp = timestamp(0);
 }
 
+void ship_level_close()
+{
+	// at this point ships have all gone through ship_delete,
+	// so don't leave any stray references behind (e.g. for sexps)
+	Ship_registry.clear();
+	Ship_registry_map.clear();
+
+	ship_close_cockpit_displays(Player_ship);
+
+	Num_wings = 0;
+}
+
 /**
  * Add a ship onto the exited ships list.
  *
@@ -8745,6 +8757,7 @@ void ship_close_cockpit_displays(ship* shipp)
 {
 	if (shipp && shipp->cockpit_model_instance >= 0) {
 		model_delete_instance(shipp->cockpit_model_instance);
+		shipp->cockpit_model_instance = -1;
 	}
 
 	for ( int i = 0; i < (int)Player_displays.size(); i++ ) {

code/ship/ship.h

@@ -1688,6 +1688,7 @@ extern SCP_vector<wing_formation> Wing_formations;
 
 extern void ship_init();				// called once	at game start
 extern void ship_level_init();		// called before the start of each level
+extern void ship_level_close();		// called after the end of each level
 
 //returns -1 if failed
 extern int ship_create(matrix* orient, vec3d* pos, int ship_type, const char* ship_name = nullptr, bool standalone_ship = false);

freespace2/freespace.cpp

@@ -984,7 +984,7 @@ void game_level_close()
 		particle::ParticleManager::get()->clearSources();
 		particle::close();
 		trail_level_close();
-		ship_close_cockpit_displays(Player_ship);
+		ship_level_close();
 		hud_level_close();
 		hud_escort_clear_all();
 		model_instance_free_all();