[sheepdog] Segfault for 0.4.0 branch

[Liu Yuan]

Got an weird segfault,

(gdb) where
#0  0x0000000000411936 in do_process_work (work=0xd13c70) at ops.c:992
#1  0x000000000040ed05 in worker_routine (arg=0xd12a20) at work.c:171
#2  0x00007f43f992c971 in start_thread (arg=<value optimized out>) at
pthread_create.c:304
#3  0x00007f43f8eeef3d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#4  0x0000000000000000 in ?? ()

sheep.log:
...
Jul 09 09:47:23 [main] client_handler(764) connection seems to be dead
Jul 09 09:47:23 [main] clear_client(703) refcnt:0, fd:14, ::1:43328
Jul 09 09:47:23 [main] destroy_client(672) connection from: ::1:43328
Jul 09 09:47:23 [main] cdrv_cpg_deliver(448) 5
Jul 09 09:47:23 [main] sd_notify_handler(851) size: 96, from: IPv4
ip:127.0.0.1 port:7000
Jul 09 09:47:23 [main] client_tx_handler(663) connection from: 13, ::1:43330
Jul 09 09:47:23 [main] client_handler(764) connection seems to be dead
Jul 09 09:47:23 [main] clear_client(703) refcnt:0, fd:13, ::1:43330
Jul 09 09:47:23 [main] destroy_client(672) connection from: ::1:43330
Jul 09 09:47:23 [main] listen_handler(819) accepted a new connection: 13
Jul 09 09:47:23 [main] listen_handler(819) accepted a new connection: 14
Jul 09 09:47:23 [block] do_process_work(990) 80, 0 , 32579 <--- XXX
Jul 09 09:47:23 [main] client_rx_handler(577) connection from: 14, ::1:43337
Jul 09 09:47:23 [main] queue_request(323) 2
Jul 09 09:47:23 [main] crash_handler(408) sheep pid 5326 exited
unexpectedly.

Thanks,
Yuan