sctp: set stream ext to NULL after freeing it in sctp_stream_outq_migrate

[ Upstream commit af98c5a78517c04adb5fd68bb64b1ad6fe3d473f ]

In sctp_stream_init(), after sctp_stream_outq_migrate() freed the
surplus streams' ext, but sctp_stream_alloc_out() returns -ENOMEM,
stream->outcnt will not be set to 'outcnt'.

With the bigger value on stream->outcnt, when closing the assoc and
freeing its streams, the ext of those surplus streams will be freed
again since those stream exts were not set to NULL after freeing in
sctp_stream_outq_migrate(). Then the invalid-free issue reported by
syzbot would be triggered.

We fix it by simply setting them to NULL after freeing.

Fixes: 5bbbbe32 ("sctp: introduce stream scheduler foundations")
Reported-by:  <syzbot+58e480e7b28f2d890bfd@syzkaller.appspotmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>