PHP5.3 fatal error when $variables['path'] is a string rather than an array

PHP5.3 returns 'true' for isset($variables['path']['path']) on line 622, even when $variables['path'] is a string ($variables['path']['path'] being evaluated as the first character in the string).

This causes a fatal error in PHP < 5.4 when the function l() is passed a string rather than an array as expected in line 626.

To fix the issue we just need to check that $variables['path'] is an array rather than a string.