Backport a "valid page" check for KSharedDataCache to KDE 4.5.3.

This adds additional checks that seem to prevent crashes when changing the system time.
This only masks the symptom though, it should be a logic error to be removing a page that is already marked as free in the code paths that could reach the affected function.

Either way, it's one less crash.

I'll leave the bug open until I feel I've figured out why the underlying cause of the bug when changing system time.

As an aside to Beat Wolf, I am just as likely (if not more) to lose patches on ReviewBoard as I am on bugs.kde.org.
Adding BugZilla attachments to ReviewBoard isn't necessary for that reason, ReviewBoard should be for things that require actual code review. ;)