default_controls.js: canvas fail fix. Misc cleanup.

- Make sure that canvas exists (i.e. didn't throw an error) before
  trying to call canvas method get_canvas_uri.

- Typos in HTML render engine debug output.

include/canvas.js

@@ -109,8 +109,8 @@ function constructor() {
 
     if (UE.gecko) { Util.Debug("Browser: gecko " + UE.gecko); }
     if (UE.webkit) { Util.Debug("Browser: webkit " + UE.webkit); }
-    if (UE.trident) { Util.Debug("Browser: webkit " + UE.trident); }
-    if (UE.presto) { Util.Debug("Browser: webkit " + UE.presto); }
+    if (UE.trident) { Util.Debug("Browser: trident " + UE.trident); }
+    if (UE.presto) { Util.Debug("Browser: presto " + UE.presto); }
 
     that.clear();
 

include/default_controls.js

@@ -247,7 +247,7 @@ settingsDisabled: function(disabled, rfb) {
     //Util.Debug(">> settingsDisabled");
     $('VNC_encrypt').disabled = disabled;
     $('VNC_true_color').disabled = disabled;
-    if (rfb && rfb.get_canvas().get_cursor_uri()) {
+    if (rfb && rfb.get_canvas() && rfb.get_canvas().get_cursor_uri()) {
         $('VNC_cursor').disabled = disabled;
     } else {
         DefaultControls.updateSetting('cursor', false);