better managed ownership (two new functions in level.h)

src/potion.cc

@@ -47,15 +47,10 @@ void potion::print(output *out) {
         out->print_char(pos, 'P', COLOR_PAIR(COLOR_GREEN));
 }
 
-potion *get_potion_at(const position &pos, potion_list &plist) {
-        potion *ret = nullptr;
-
+size_t get_potion_at(const position &pos, const potion_list &plist) {
         for (size_t i = 0; i < plist.size(); ++i)
-                if (plist[i]->get_pos() == pos) {
-                        ret = plist[i];
-                        plist.erase(plist.begin() + i);
-                        return ret;
-                }
+                if (plist[i]->get_pos() == pos)
+                        return i;
 
-        return ret;
+        return plist.size();
 }