sctp: use a valid outstream when adding it to the scheduler

Without holding the stcb send lock, the outstreams might get
reallocated if the number of streams are increased.

Reported by: syzbot+4a5431d7caa666f2c19c@syzkaller.appspotmail.com
Reported by: syzbot+aa2e3b013a48870e193d@syzkaller.appspotmail.com
Reported by: syzbot+e4368c3bde07cd2fb29f@syzkaller.appspotmail.com
Reported by: syzbot+fe2f110e34811ea91690@syzkaller.appspotmail.com
Reported by: syzbot+ed6e8de942351d0309f4@syzkaller.appspotmail.com
MFC after: 1 week