apps/dgst.c: Set digestname from argv[0] if it is a builtin hash name

Fixes #19589

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/19606)

apps/dgst.c

@@ -114,6 +114,8 @@ int dgst_main(int argc, char **argv)
 
     buf = app_malloc(BUFSIZE, "I/O buffer");
     md = (EVP_MD *)EVP_get_digestbyname(argv[0]);
+    if (md != NULL)
+        digestname = argv[0];
 
     opt_set_unknown_name("digest");
     prog = opt_init(argc, argv, dgst_options);