libnvdimm, btt: check memory allocation failure

Check memory allocation failures and return -ENOMEM in such cases, as
already done few lines below for another memory allocation.

This avoids NULL pointers dereference.

Cc: <stable@vger.kernel.org>
Fixes: 14e49454 ("libnvdimm, btt: BTT updates for UEFI 2.7 format")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>