Fix disconnect crash when screen doesn't exist

The function removeChild throws an exception when the element specified
doesn't exist. This can happen when opening the websocket throws an
exception during connect. Fixes issue #979

core/rfb.js

@@ -433,7 +433,16 @@ RFB.prototype = {
         this._mouse.ungrab();
         this._sock.close();
         this._print_stats();
-        this._target.removeChild(this._screen);
+        try {
+            this._target.removeChild(this._screen);
+        } catch (e) {
+            if (e.name === 'NotFoundError') {
+                // Some cases where the initial connection fails
+                // can disconnect before the _screen is created
+            } else {
+                throw e;
+            }
+        }
         clearTimeout(this._resizeTimeout);
         Log.Debug("<< RFB.disconnect");
     },