sctp: improve computation of an alternate net

Espeially handle the case where the net passed in is about to
be deleted and therefore not in the list of nets anymore.

Reported by: syzbot+9756917a7c8381adf5e8@syzkaller.appspotmail.com

(cherry picked from commit b963ce4588b33f733aef3f7a7f3fbe5018a89728)