Fix undefined check.

interface_config.js

     // the toolbar buttons line is intentionally left in one line, to be able
     // to easily override values or remove them using regex
     MAIN_TOOLBAR_BUTTONS: ['microphone', 'camera', 'desktop', 'invite', 'fullscreen', 'hangup'], // jshint ignore:line
-    MAIN_TOOLBAR_SPLITTER_INDEX: -1,
+    /**
+     * The index of the splitter button in the main toolbar. The splitter
+     * button is a button in the toolbar that will be applied a special styling
+     * visually dividing the toolbar buttons.
+     */
+    //MAIN_TOOLBAR_SPLITTER_INDEX: -1,
     TOOLBAR_BUTTONS: ['profile', 'authentication', 'microphone', 'camera', 'desktop', 'recording', 'security', 'raisehand', 'chat', 'etherpad', 'sharedvideo', 'sip', 'dialpad', 'settings', 'hangup', 'filmstrip', 'contacts'], // jshint ignore:line
     SETTINGS_SECTIONS: ['language', 'devices', 'moderator'],
     // Determines how the video would fit the screen. 'both' would fit the whole

modules/UI/toolbars/Toolbar.js

                     button,
                     (index === 0),
                     (index === interfaceConfig.MAIN_TOOLBAR_BUTTONS.length -1),
-                    (interfaceConfig.MAIN_TOOLBAR_SPLITTER_INDEX
+                    (interfaceConfig.MAIN_TOOLBAR_SPLITTER_INDEX !== undefined
                         && index
                             === interfaceConfig.MAIN_TOOLBAR_SPLITTER_INDEX));
             }