Add missing refcount increment for systemlist()

- get_system_output_as_rettv() was missing a refcount increment when
  returning an empty list, i.e. when there was no output
- we now use rettv_list_aloc() instead of list_alloc()
- issue #1530

src/nvim/eval.c

@@ -14588,8 +14588,7 @@ static void get_system_output_as_rettv(typval_T *argvars, typval_T *rettv,
   if (res == NULL) {
     if (retlist) {
       // return an empty list when there's no output
-      rettv->v_type = VAR_LIST;
+      rettv_list_alloc(rettv);
-      rettv->vval.v_list = list_alloc();
     }
     return;
   }