Running file_exists on a URL prefixed with "//" takes forever to run

This might be Windows-related. When a filepath is prefixed with "//", Windows attempts to locate a network share with that address (which I'm guessing is the bottleneck).

This patch skips the `file_exists` check if the $script starts with "//". There's probably a more elegant fix, such as using is_ssl() to force either "http" or "https".