guard against index worker rpc responses for a txid with no supplied callback

lib/hk-util.js

@@ -835,7 +835,14 @@ HK.initializeIndexWorkers = function (Env, config, _cb) {
         worker.on('message', function (res) {
             if (!res || !res.txid) { return; }
             //console.log(res);
+            try {
                 response.handle(res.txid, [res.error, res.value]);
+            } catch (err) {
+                Env.Log.error("INDEX_WORKER", {
+                    error: err,
+                    response: res,
+                });
+            }
         });
         worker.on('exit', function () {
             var idx = workers.indexOf(worker);