Improve const correctness for stacks of EVP_MD

EVP_MDs are always const, so stacks of them should be too. This silences
a warning about type punning on OpenBSD.

RT4378
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/ts/ts_rsp_sign.c

@@ -223,7 +223,7 @@ int TS_RESP_CTX_add_md(TS_RESP_CTX *ctx, const EVP_MD *md)
     if (ctx->mds == NULL
         && (ctx->mds = sk_EVP_MD_new_null()) == NULL)
         goto err;
-    if (!sk_EVP_MD_push(ctx->mds, (EVP_MD *)md))
+    if (!sk_EVP_MD_push(ctx->mds, md))
         goto err;
 
     return 1;
@@ -446,7 +446,7 @@ static int ts_RESP_check_request(TS_RESP_CTX *ctx)
     X509_ALGOR *md_alg;
     int md_alg_id;
     const ASN1_OCTET_STRING *digest;
-    EVP_MD *md = NULL;
+    const EVP_MD *md = NULL;
     int i;
 
     if (TS_REQ_get_version(request) != 1) {
@@ -460,7 +460,7 @@ static int ts_RESP_check_request(TS_RESP_CTX *ctx)
     md_alg = msg_imprint->hash_algo;
     md_alg_id = OBJ_obj2nid(md_alg->algorithm);
     for (i = 0; !md && i < sk_EVP_MD_num(ctx->mds); ++i) {
-        EVP_MD *current_md = sk_EVP_MD_value(ctx->mds, i);
+        const EVP_MD *current_md = sk_EVP_MD_value(ctx->mds, i);
         if (md_alg_id == EVP_MD_type(current_md))
             md = current_md;
     }

include/openssl/ts.h

@@ -306,7 +306,7 @@ typedef int (*TS_extension_cb) (struct TS_resp_ctx *, X509_EXTENSION *,
 
 typedef struct TS_resp_ctx TS_RESP_CTX;
 
-DEFINE_STACK_OF(EVP_MD)
+DEFINE_STACK_OF_CONST(EVP_MD)
 
 /* Creates a response context that can be used for generating responses. */
 TS_RESP_CTX *TS_RESP_CTX_new(void);