[FIX] tablesorter: Some browsers (e.g. Firefox) don't have a global event var, so check for that first otherwise the data doesn't load.
Means the bug still occurs but at the least the table is usable, albeit looking semitransparent (thanks Lindon :)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@52656 b456876b-0849-0410-b77d-98878d47e9d5