Handle possible race condition with `mkdir`

[arogachev]

https://github.com/kalessil/phpinspectionsea/blob/master/docs/probable-bugs.md#mkdir-race-condition

Extracted from comment:

https://github.com/yiisoft/rbac-php/blob/cb61807d57945f7ab6b930bc8a3b343f0787eb46/src/CommonStorage.php#L48-L62