sctp: don't keep a pointer to a freed stcb around
Reported by: syzbot+b9ef06efdae7cb9ee414@syzkaller.appspotmail.com
Reported by: syzbot+b1e4793e0e6b25b0d510@syzkaller.appspotmail.com
(cherry picked from commit eeba222172170c380450004fdceac80c3612b1e8)