Avoid duplicate processing of menu items

Problem/Motivation

Under the right conditions (see below for an example), the "admin/config/content/panelizer/node/[bundle].default/list" page will return a blank page and generate a bunch of errors (not displayed of course). The next successful page load will show the errors, like:

• Warning: array_key_exists(): The first argument should be either a string or an integer in ctools_get_plugins() (line 240 of /ctools/includes/plugins.inc).
• Warning: array_key_exists(): The first argument should be either a string or an integer in ctools_get_plugins() (line 331 of /ctools/includes/plugins.inc).
• Warning: array_key_exists(): The first argument should be either a string or an integer in ctools_get_plugins() (line 341 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in isset or empty in ctools_plugin_load_includes() (line 489 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in isset or empty in ctools_plugin_load_includes() (line 489 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in isset or empty in ctools_plugin_load_includes() (line 489 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in isset or empty in ctools_plugin_load_includes() (line 489 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in isset or empty in ctools_get_plugins() (line 344 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in ctools_get_plugins() (line 344 of /ctools/includes/plugins.inc).
• Warning: Illegal offset type in ctools_get_plugins() (line 354 of /ctools/includes/plugins.inc).
• Warning: Invalid argument supplied for foreach() in panelizer_export_ui_switcher_page() (line 748 of /panelizer/panelizer.module).
• Notice: Undefined index: redirect in panelizer_export_ui_switcher_page() (line 755 of /panelizer/panelizer.module).
• Warning: Invalid argument supplied for foreach() in panelizer_export_ui_switcher_page() (line 755 of /panelizer/panelizer.module).

The errors result from an invalid menu router item produced by executing panelizer_defaults_ui::hook_menu() twice on the same set of menu items. The page arguments are invalid and the page callback (panelizer_export_ui_switcher_page()) responds with a blank page.

To reproduce the problem, install a site with panelizer and pathauto enabled. The invalid menu router item will exist after submitting the install_configure_form (where you set the administrative user name and password, etc.). Enable panelizer on a content type and attempt to view the "list" URL mentioned above.

Proposed resolution

Add a condition in panelizer_defaults_ui::hook_menu to check whether a menu item has been "processed" by panelizer; if it has do not process it again.

Remaining tasks

Review and test.

User interface changes

None.

API changes

None.