[Owncloud] Stable4: Error message on creating user on calendar table

[Arthur Schiwon]

Hey,

with current stable4 the following warning is being displayed after when 
creating a new user (using PostgreSQL, did not test with sqlite/mysql).

SQLSTATE[23505]: Unique violation: 7 ERROR: duplicate key value violates 
unique constraint "oc_calendar_calendars_pkey" DETAIL: Key (id)=(0) 
already exists.

Cheers
Arthur