crypto/sm2/sm2_sign.c: Add BN_CTX_end

To match the BN_CTX_start, it should be better to add
BN_CTX_end in the end of the function.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #19266)

crypto/sm2/sm2_sign.c

@@ -390,6 +390,7 @@ static int sm2_sig_verify(const EC_KEY *key, const ECDSA_SIG *sig,
         ret = 1;
 
  done:
+    BN_CTX_end(ctx);
     EC_POINT_free(pt);
     BN_CTX_free(ctx);
     return ret;