[FIX] trackers: Prevent list_items from exhausting memory completely and just return whatever items have been retrieved so far when we're down to 10MB left.
CAn happen when exporting large trackers leading to just the header being exported.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/11.x@46546 b456876b-0849-0410-b77d-98878d47e9d5