hash_job_done: set hash_data->error to NULL when cleaning it
hash_job_done: set hash_data->error to NULL when cleaning it

This avoid to try to clean it at second time in hash_data_free (#606220).

Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets