Correctly terminate if no config file can be found

If no config file is found pk_util_get_config_filename returns
NULL, which then gets passed into g_key_file_load_from_file
which if it receives a NULL value as a filename it will return
FALSE, and not set error, causing a segfault when trying to
print error->message.


I'm not sure if you take pull requests here, but I figured I'd give it a shot.