media: pci: cx18: if cx == NULL, then don't use it.

If the cx NULL pointer check is true, then it reports
an error with CX18_ERR, but that uses cx. Replace it with
pr_err.

Reported by smatch:

drivers/media/pci/cx18/cx18-mailbox.c:834 cx18_vapi() error: we previously assumed 'cx' could be null (see line 833)

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>