Fix Return Value `FileAttributeKey::getByHandle()`

Fix return value for `FileAttributeKey::getByHandle()` where method will
return `-1` on initial failure to find object by handle. On following
attempts `false` will be returned

- Change to return `false` when `$ak === -1`
Merge pull request #1016 from mkly/fix_file_attribute_key_return_value

Fix Return Value `FileAttributeKey::getByHandle()`