Fix bug with stonewall option causing segfault

Fix bug with stonewall option causing segfault

fio core dumped , with stonewall,

[global]
ioengine=libaio
direct=1
bs=8k
iodepth=1024
iodepth_batch=60
randrepeat=1
size=1024m
directory=/home/oracle
[job1]
rw=randread
filename=file1
[job2]
rw=randwrite
filename=file1

stonewall

[job3]
rw=randread
filename=file1
[job4]
rw=randwrite
filename=file1

 >fio myjob
job1: (g=0): rw=randread, bs=8K-8K/8K-8K, ioengine=libaio, iodepth=1024
job2: (g=1): rw=randwrite, bs=8K-8K/8K-8K, ioengine=libaio, iodepth=1024
job3: (g=1): rw=randread, bs=8K-8K/8K-8K, ioengine=libaio, iodepth=1024
job4: (g=1): rw=randwrite, bs=8K-8K/8K-8K, ioengine=libaio, iodepth=1024
Starting 4 processes
Quit (core dumped)PP] [99.3% done] [  1298/     0 kb/s] [eta 00m:03s]

(gdb) bt
t_rate=0xfeeee790, m_rate=0xfeeee794) at fio.c:1011

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>