mtd: core: Potential NULL dereference in mtd_otp_size()

If kmalloc() fails then it could lead to a NULL dereference.  Check and
return -ENOMEM on error.

Fixes: 4b361cfa ("mtd: core: add OTP nvmem provider support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/YJ6Iw3iNvGycAWV6@mwanda