fix possible eina file shutdown issue

i am not sure as i cannto reproduce this, but i hope this fixes T4677
by ensuring if eina_file_shutdown is called it cannto double-free a
hash.

src/lib/eina/eina_file_common.c

@@ -1054,6 +1054,7 @@ eina_file_shutdown(void)
      }
 
    eina_hash_free(_eina_file_cache);
+   _eina_file_cache = NULL;
 
    eina_lock_free(&_eina_file_lock_cache);