[SCSI] drivers/scsi/sr_ioctl.c: check for failed allocation

I noticed a case in sr_ioctl.c's sr_get_mcn where a buffer is
allocated, but the pointer isn't checked for null.

Signed-off-by: Nate Dailey <nate.dailey@stratus.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>