pktcdvd: check for NULL returna fter calling bio_split_to_limits()

The revert of the removal of this driver happened after we fixed up
the split limits for NOWAIT issue, hence it got missed. Ensure that
we check for a NULL bio after splitting, in case it should be retried.

Marking this as fixing both commits, so that stable backport will do
this correctly.

Cc: stable@vger.kernel.org
Fixes: 9cea62b2 ("block: don't allow splitting of a REQ_NOWAIT bio")
Fixes: 4b83e99e ("Revert "pktcdvd: remove driver."")
Signed-off-by: Jens Axboe <axboe@kernel.dk>