wp_guess_url() calls strpos() incorrectly if $_SERVER['SCRIPT_FILENAME'] is empty

[bjorsch]

This can happen, for example, if using PHPUnit's @runInSeparateProcess.

The message produced is

PHP Warning:  strpos(): Empty needle in /var/www/html/wp-includes/functions.php on line 5435

In the aforementioned PHPUnit case, that warning causes tests to fail.

[bjorsch]

Patch to check for empty $script_filename_dir before passing it to strpos()

[kraftbj]

Introduced in r25436.