lib/mpi: Add the return value check of kcalloc()

Add the return value check of kcalloc() to avoid potential
NULL ptr dereference.

Fixes: a8ea8bdd ("lib/mpi: Extend the MPI library")
Signed-off-by: Zizhuang Deng <sunsetdzz@gmail.com>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>