create_post() causes db error when non-existent user is subscribed to a thread.

[TJM]

If user id exists in [b]subscriptionsb table but doesn't exist in [b]userb table (happened on my project after I've deleted spammers accounts) create_post() causes db erros, because it tries to execute query 'query: replace into enigma.notify set [b]userid=b, create_time=1223811818, type=4, opaque=2' which fails.

Solution: check if user exists before calling notify_subscriber(), if it doesn't then remove entry from subscriptions table.