Fix fetch crash when all parts come back immediately

Resolves "TypeError: Cannot set property 'ended' of undefined" if
bodies are sent immediately and no msg is added to the fetchCache.

lib/Connection.js

@@ -1115,7 +1115,7 @@ Connection.prototype._resUntagged = function(info) {
 
     if (toget.length === 0) {
       process.nextTick(function() {
-        msg.ended = true;
+        if (msg) msg.ended = true;
         msgEmitter.emit('attributes', attrs);
         msgEmitter.emit('end');
       });