sctp: fix double unlock in case adding a remote address fails

Thanks to glebius@ for pointing to the problem.

Reported by: syzbot+1d5c164f1c10de84ad8a@syzkaller.appspotmail.com
Fixes: 2d5c48eccd9f ("sctp: Tighten up locking around sctp_aloc_assoc()"
MFC after: 3 days