Update sm2_crypt.c

asn1_encode : x, y  =>    0 | x，0 | y
(because of DER encoding rules when x and y have high bit set)

CLA: Trivial
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6694)

crypto/sm2/sm2_crypt.c

@@ -95,7 +95,7 @@ int sm2_ciphertext_size(const EC_KEY *key, const EVP_MD *digest, size_t msg_len,
     if (field_size == 0 || md_size < 0)
         return 0;
 
-    *ct_size = 10 + 2 * field_size + (size_t)md_size + msg_len;
+    *ct_size = 12 + 2 * field_size + (size_t)md_size + msg_len;
     return 1;
 }