pktcdvd: check for NULL returna fter calling bio_split_to_limits()

commit 3e9900f3 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>