sctp: clear pointer to stack when returning from function.

Reported by: syzbot+04cee5d8805dfbb63c06@syzkaller.appspotmail.com
Reported by: syzbot+71e7e33dfc3cc39a6bd0@syzkaller.appspotmail.com
Reported by: syzbot+6c36fc3c1bd03ed96107@syzkaller.appspotmail.com
Reported by: syzbot+198b3751c158181c47de@syzkaller.appspotmail.com

(cherry picked from commit 39a22011bbb820b389ce8ad166f7bdce112a75cc)