coverity 1462564 Improper use of negative value

Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11651)

providers/implementations/macs/kmac_prov.c

@@ -250,6 +250,8 @@ static int kmac_init(void *vmacctx)
         return 0;
 
     block_len = EVP_MD_block_size(ossl_prov_digest_md(&kctx->digest));
+    if (block_len < 0)
+        return 0;
 
     /* Set default custom string if it is not already set */
     if (kctx->custom_len == 0) {