Make nullplugin dialog not show up on clicks with buttons other than the left

mouse button.  Bug 270377, patch by Vincent Caron <v.caron@zerodeux.net>,
r=bryner, sr=bzbarsky

modules/plugin/samples/default/unix/nullplugin.c

@@ -467,7 +467,10 @@ xt_event_handler(Widget xt_w, PluginInstance *This, XEvent *xevent, Boolean *b)
             drawPixmap(This);
             break;
         case ButtonRelease:
-            makeWidget(This);
+            if (xevent->xbutton.button == Button1)
+            {
+                makeWidget(This);
+            } 
             break;
         default:
             break;