zbd: avoid failing assertion in zbd_convert_to_open_zone()

zbd: avoid failing assertion in zbd_convert_to_open_zone()

The test run against null_blk with the following command line -

t/zbd/run-tests-against-nullb -l -q -s 12 -t 51 -n 100

stops with a failure and the message below can be seen in the test log:

fio: zbd.c:1110: zbd_convert_to_open_zone: Assertion `open_zone_idx < f->zbd_info->num_open_zones' failed.

This assertion fails because pick_random_zone_idx() function returns
index 0 if no zones are currently open. In this case, open_zone_idx and
f->zbd_info->num_open_zones are both zero. Since this situation is
normal, simply modify the assert statement to avoid failing.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>