Installer Class missing array variable check

Fixes:
PHP Warning:  substr() expects parameter 1 to be string, array given in /var/www/openemr/library/classes/Installer.class.php on line 311, referer: http://openemr.example.com/setup.php?site=default

Explained in Forum Post:
https://sourceforge.net/p/openemr/discussion/202506/thread/848c1507/