Fix a bug where messages would not be sent in SHUTDOWN_RECEIVED state.
This problem was reported by Mark Bonnekessel and Markus Boese.
Thanks to Irene Ruengeler for helping me to fix the cause of
the problem. It can be tested with the following packetdrill script:

+0.0 socket(..., SOCK_STREAM, IPPROTO_SCTP) = 3
+0.0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
+0.0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
Check the handshake with an empty(!) cookie
+0.1 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0.0 > sctp: INIT[flgs=0, tag=1, a_rwnd=..., os=..., is=..., tsn=0, ...]
+0.1 < sctp: INIT_ACK[flgs=0, tag=2, a_rwnd=10000, os=1, is=1, tsn=0, STATE_COOKIE[len=4, val=...]]
+0.0 > sctp: COOKIE_ECHO[flgs=0, len=4, val=...]
+0.1 < sctp: COOKIE_ACK[flgs=0]
+0.0 getsockopt(3, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
+0.0 write(3, ..., 1024) = 1024
+0.0 > sctp: DATA[flgs=BE, len=1040, tsn=0, sid=0, ssn=0, ppid=0]
+0.0 write(3, ..., 1024) = 1024 Pending due to Nagle
+0.0 < sctp: SHUTDOWN[flgs=0, cum_tsn=0]
+0.0 > sctp: DATA[flgs=BE, len=1040, tsn=1, sid=0, ssn=1, ppid=0]
+0.0 < sctp: SACK[flgs=0, cum_tsn=1, a_rwnd=10000, gaps=[], dups=[]] // Do we need another SHUTDOWN here?
+0.0 > sctp: SHUTDOWN_ACK[flgs=0]
+0.0 < sctp: SHUTDOWN_COMPLETE[flgs=0]
+0.0 close(3) = 0

MFC after: 3 days