net: sctp: fix NULL pointer dereference in socket destruction

      While stress testing sctp sockets, I hit the following panic:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
IP: [<ffffffffa0490c4e>] sctp_endpoint_free+0xe/0x40 [sctp]
PGD 7cead067 PUD 7ce76067 PMD 0
Oops: 0000 [#1] SMP
Modules linked in: sctp(F) libcrc32c(F) [...]
CPU: 7 PID: 2950 Comm: acc Tainted: GF            3.10.0-rc2+ #1
Hardware name: Dell Inc. PowerEdge T410/0H19HD, BIOS 1.6.3 02/01/2011
task: ffff88007ce0e0c0 ti: ffff88007b568000 task.ti: ffff88007b568000
RIP: 0010:[<ffffffffa0490c4e>]  [<ffffffffa0490c4e>] sctp_endpoint_free+0xe/0x40 [sctp]
RSP: 0018:ffff88007b569e08  EFLAGS: 00010292
RAX: 0000000000000000 RBX: ffff88007db78a00 RCX: dead000000200200
RDX: ffffffffa049fdb0 RSI: ffff8800379baf38 RDI: 0000000000000000
RBP: ffff88007b569e18 R08: ffff88007c230da0 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffff880077990d00 R14: 00000000000000