fixed segmentation fault when sorting enemies

src/game.cc

@@ -41,17 +41,26 @@ void game::new_level() {
                                  rng, enabled_features));
 }
 
-#include <algorithm>
-bool compare_characters(const character *a, const character *b) {
-        return a->get_pos() < b->get_pos();
+enemy_list game::sort_enemies(const enemy_list &elist) {
+        enemy_list result;
+        result.reserve(elist.size());
+
+        for (auto enemy : elist) {
+                result.push_back(enemy);
+
+                for (int i = result.size() - 1; i > 0 &&
+                     enemy->get_pos() < result[i - 1]->get_pos(); --i)
+                        std::swap(result[i], result[i - 1]);
+        }
+
+        return result;
 }
 
 character *game::move_enemies() {
         if (the_world)
                 return nullptr;
 
-        auto enemies = levels[curr_level]->get_elist();
-        std::sort(enemies.begin(), enemies.end(), ::compare_characters);
+        auto enemies = sort_enemies(levels[curr_level]->get_elist());
 
         for (auto ch : enemies) {
                 bool hostile = ch->get_race() == rmerchant ?