Fix file cache engine trying to clear root on realpath failure.
Fix file cache engine trying to clear root on realpath failure.

When `getRealPath()` fails and returns `false`, the path will finally
contain only the appended directory separator, thus pointing to the root
directory.

ADmad/cakephp,ADmad/cakephp,CakeDC/cakephp,cakephp/cakephp,cakephp/cakephp,ADmad/cakephp,CakeDC/cakephp,CakeDC/cakephp,cakephp/cakephp,cakephp/cakephp,ndm2/cakephp,ADmad/cakephp,ndm2/cakephp,ndm2/cakephp,ndm2/cakephp,CakeDC/cakephp