fix access to array element by using the one that was checked for

Just played around with it and tried to get only the minimum file set into my resulting phar, therefore tried to use the `ExplicitBundler` with my dependencies. One of these has files defined, which currently results in a warning when this line is executed:

    Warning: Invalid argument supplied for foreach() in phar:///path/to/phar-composer.phar/src/Clue/PharComposer/Bundler/Explicit.php on line 58