Constify CMS_decrypt_set1_key input buffer

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1300)

crypto/cms/cms_smime.c

@@ -672,7 +672,7 @@ int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert)
 
 int CMS_decrypt_set1_key(CMS_ContentInfo *cms,
                          unsigned char *key, size_t keylen,
-                         unsigned char *id, size_t idlen)
+                         const unsigned char *id, size_t idlen)
 {
     STACK_OF(CMS_RecipientInfo) *ris;
     CMS_RecipientInfo *ri;

include/openssl/cms.h

@@ -144,7 +144,7 @@ int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert,
 int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert);
 int CMS_decrypt_set1_key(CMS_ContentInfo *cms,
                          unsigned char *key, size_t keylen,
-                         unsigned char *id, size_t idlen);
+                         const unsigned char *id, size_t idlen);
 int CMS_decrypt_set1_password(CMS_ContentInfo *cms,
                               unsigned char *pass, ossl_ssize_t passlen);