iolog: Fix write_iolog_close()

iolog: Fix write_iolog_close()

If the init_iolog() call from backend.c thread_main() fails (e.g. wrong
file path given), td->iolog_f is not set but write_iolog_close() is
still called from thread_main() error processing. This causes a seg
fault and unclean termination of fio. Fix this by changing
write_iolog_close() to do nothing if td->iolog_f is NULL.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>