We previously did this only in the normal case where no association
exists yet. However, it is not safe to process COOKIE-ECHO even if an
association exists, as sctp_process_cookie_existing() may dereference
the socket pointer.
See also commit 0c7dc84076b64ef74c24f04400d572f75ef61bb4. I'm not 100%
sure that this is the right fix.