Check the plaintext buffer is large enough when decrypting SM2

Check the plaintext buffer is large enough when decrypting SM2

Previously there was no check that the supplied buffer was large enough.
It was just assumed to be sufficient. Instead we should check and fail if
not.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>