add_message: Avoid a memory leak when user holds on to message return.

When this function was originally written, the 'message' object was
always destroyed locally, so I thought it would be good to use a NULL
talloc context to make it more obvious if there was any leak.

Since then, however, this function has been changed to optionally
return the added message, and in that case we *don't* free the message
locally, so let's let the database be the talloc context.