guard against the possible non-existence of metadata for a channel

lib/hk-util.js

@@ -955,7 +955,7 @@ HK.onChannelMessage = function (Env, Server, channel, msgStruct) {
         }));
     }).nThen(function (w) {
         // if there's no validateKey present skip to the next block
-        if (!metadata.validateKey) { return; }
+        if (!(metadata && metadata.validateKey)) { return; }
 
         // trim the checkpoint indicator off the message if it's present
         let signedMsg = (isCp) ? msgStruct[4].replace(CHECKPOINT_PATTERN, '') : msgStruct[4];