ringaccount: prevent adding invalid contact

Use of an invalid ringId could lead to adding
an invalid contact to the account.
Prevent this with a check in addContact and updateContact

Change-Id: I2e396425cf0283e63c2262d8aca6724edf1d9f17
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>