get_plugins_with_function needs to check if function exists

1) Edit admin/tool/usertours/lib.php and rename the tool_usertours_extend_navigation_users function to be incorrect: tool_usertourse_extend_navigation_user*z* 2) Visit the dashboard, and verify the page loads with no errors. 3) Undo the change to the function. 4) Create a file called test.php in the root directory, and add the following content to it. <?php require_once( 'config.php' ); $functions = get_plugin_list_with_function( 'tool' , 'extend_navigation_user' ); var_dump(array_key_exists( 'tool_usertours' , $functions)); 5) Purge caches 6) Visit the dashboard and let the page load. 7) Change the name of the usertours function to be incorrect again: tool_usertours_extend_navigation_user*z* 8) Visit the YOURSITE/test.php 9) Verify you see false on the page. This shows that the function is no longer in the returned cached function list.