[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>
diff --git a/drivers/scsi/sr_ioctl.c b/drivers/scsi/sr_ioctl.c
index 3471be0..82d68fd 100644
--- a/drivers/scsi/sr_ioctl.c
+++ b/drivers/scsi/sr_ioctl.c
@@ -281,6 +281,9 @@
 	char *buffer = kmalloc(32, GFP_KERNEL | SR_GFP_DMA(cd));
 	int result;
 
+	if (!buffer)
+		return -ENOMEM;
+
 	memset(&cgc, 0, sizeof(struct packet_command));
 	cgc.cmd[0] = GPCMD_READ_SUBCHANNEL;
 	cgc.cmd[2] = 0x40;	/* I do want the subchannel info */