Add a XXX comment.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@4779 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/digest/digest.c

@@ -73,6 +73,7 @@ rb_digest_base_alloc(klass)
 
     algo = get_digest_base_metadata(klass);
 
+    /* XXX: An uninitialized buffer leads ALGO_Equal() to fail */
     pctx = xcalloc(algo->ctx_size, 1);
     algo->init_func(pctx);