sctp: clear out_curr if all frag chunks of current msg are pruned

In the Linux kernel, the following vulnerability has been resolved:

sctp: clear out_curr if all frag chunks of current msg are pruned

A crash was reported by Zhen Chen:

list_del corruption, ffffa035ddf01c18->next is NULL
WARNING: CPU: 1 PID: 250682 at lib/list_debug.c:49 __list_del_entry_valid+0x59/0xe0
RIP: 0010:__list_del_entry_valid+0x59/0xe0
Call Trace:
sctp_sched_dequeue_common+0x17/0x70 [sctp]
sctp_sched_fcfs_dequeue+0x37/0x50 [sctp]
sctp_outq_flush_data+0x85/0x360 [sctp]
sctp_outq_uncork+0x77/0xa0 [sctp]
sctp_cmd_interpreter.constprop.0+0x164/0x1450 [sctp]
sctp_side_effects+0x37/0xe0 [sctp]
sctp_do_sm+0xd0/0x230 [sctp]
sctp_primitive_SEND+0x2f/0x40 [sctp]
sctp_sendmsg_to_asoc+0x3fa/0x5c0 [sctp]
sctp_sendmsg+0x3d5/0x440 [sctp]
sock_sendmsg+0x5b/0x70

and in sctp_sched_fcfs_dequeue() it dequeued a chunk from stream
out_curr outq while this outq was empty.

Normally stream->out_curr must be set to NULL once all frag chunks of
current msg are dequeued, as we can see in sctp_sched_dequeue_done().
However, in sctp_prsctp_prune_unsent() as it is not a proper dequeue,
sctp_sched_dequeue_done() is not called to do this.

This patch is to fix it by simply setting out_curr to NULL when the
last frag chunk of current msg is dequeued from out_curr stream in
sctp_prsctp_prune_unsent().