Fix another verify segfault

Fix another verify segfault

If you run this job:

[global]
ioengine=libaio
direct=1
filename=/tmp/foo
iodepth=128
size=10M
loops=1
group_reporting=1
readwrite=write
do_verify=0
verify=md5
numjobs=1
thread
verify_dump=1

[small_writes]
offset=0G
blocksize=512
verify_interval=1M

[large_writes]
stonewall
offset=1G
blocksize=1M
verify_interval=512

fio crashes because verify_interval is larger than the block size
in the small_writes job. Check for this in get_hdr_inc().

Signed-off-by: Jens Axboe <axboe@kernel.dk>