Ensure that job name is set before being used

Andreas reports:

When passing the --readonly option, with the following write job (and
probably with any write job), I get an error message with a NULL job
name. Job description used:

[write]
thread
bs=64k
direct=1
ioengine=sync
size=128m
filename=test.tmp
rw=write
do_verify=0

Reported-by: Andreas Grünbacher <andreas.gruenbacher@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>