Filesize errors caused by unlink

We're seeing some issues in beta8 with filesize errors:

Warning: filesize() [function.filesize]: stat failed for /nfs/silo2/v1/drupaldev/drupal_theme_tester/sites/default/cache/cache/8/87cd8b8808600624d8c590cfc2e6e94b--variables in /nfs/disk/data/www/httpd-docs/tools/drupal/6.10_200904200200/sites/all/modules/cacherouter/engines/file.php on line 41

Warning: fread() [function.fread]: Length parameter must be greater than 0 in /nfs/disk/data/www/httpd-docs/tools/drupal/6.10_200904200200/sites/all/modules/cacherouter/engines/file.php on line 41

We've fixed the issue. It was due to filesize() not taking the file pointer and instead using the file name. Since the file was unlinked, filesize() errors, and then fread() tries to read a length of 0.

See the attached patch. It corrects the issue for get() and purge().