Add safeguards for invalidated model instances and texture replace specifically

Author: BMagnu

code/model/modelrender.cpp

@@ -3195,8 +3195,12 @@ void model_render_set_wireframe_color(const color* clr)
 
 void modelinstance_replace_active_texture(polymodel_instance* pmi, const char* old_name, const char* new_name)
 {
-	Assert(pmi != nullptr);
+	if (pmi == nullptr)
+		return;
+
 	polymodel* pm = model_get(pmi->model_num);
+	if (pm == nullptr)
+		return;
 
 	int final_index = -1;
 

code/ship/ship.cpp

@@ -8986,6 +8986,7 @@ void ship_delete( object * obj )
 	ct_ship_delete(shipp);
 
 	model_delete_instance(shipp->model_instance_num);
+	shipp->model_instance_num = -1;
 
 	// free up any weapon model instances
 	for (int i = 0; i < shipp->weapons.num_primary_banks; ++i)