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

[TJM]

If user id exists in subscriptions table but doesn't exist in user 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 userid=, 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.

[Ageless]

Fixed formatting.