Minor typo, coding style fixes for OC_Util::getInstanceId
Ensure instanceid contains a letter

instanceid is generated by uniqid() and then used as the
session_name. Because session_name requires at least one letter
and uniqid() does not guarantee to provide that, in the case that
uniqid() generates a string of only digits, the user will be stuck
in an infinite login loop because every request will generate a
new PHP session.
Merge pull request #2584 from mrtorrent/fix_instanceid_format

Fix instanceid format to prevent session loop