efreet_desktop: fix potential error in efreet_desktop_cache_create

Summary:
 - old_file_ids is freed but not set as NULL.
   If it goes to error code, old_file_ids will be freed again.

Reviewers: jpeg, cedric, Hermet

Reviewed By: Hermet

Subscribers: conr2d

Differential Revision: https://phab.enlightenment.org/D4467

src/bin/efreet/efreet_desktop_cache_create.c

@@ -504,6 +504,7 @@ main(int argc, char **argv)
     {
         eina_hash_free(old_file_ids->hash);
         free(old_file_ids);
+        old_file_ids = NULL;
     }
 
     eina_hash_free(file_ids);