crypto: sm2 - Fix crash caused by uninitialized context

In sm2_compute_z_digest() function, the newly allocated structure
mpi_ec_ctx is used, but forget to initialize it, which will cause
a crash when performing subsequent operations.

Fixes: e5221fa6

 ("KEYS: asymmetric: Move sm2 code into x509_public_key")
Cc: stable@vger.kernel.org # v6.5
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>