Silence PHP warnings from disk_free_space(). disk_free_space() will produce a warning in error conditions in addition to returning false, this includes a case where the bytes free is greater than PHP_INT_MAX (which is a error condition we don't need to check).
See #25576, #22704


git-svn-id: https://develop.svn.wordpress.org/trunk@25831 602fd350-edb4-49c9-b593-d223f7449a82