Stabilize a condition
Stabilize a condition

While looking into the race condition described in 125, I noticed a potentially related issue.

As per the documentation of [realpath()](https://www.php.net/manual/en/function.realpath.php#refsect1-function.realpath-returnvalues):
> realpath() returns FALSE on failure, e.g. if the file does not exist.

In the rare case that `realpath()` _would_ return `false` within this plugin, `false` would subsequently be passed on to `is_dir()` and `is_readable()` which both expect a string path.

This tiny change make the condition fail earlier by checking the return value of `realpath()`.

DealerDirect/phpcodesniffer-composer-installer