[FIX] trackers: Validate user really exists before storing new value. Prevents fatal error when option->notify is enabled for the field (thanks Torsten for the example, and Nelson for the diagnosis ;)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48278 b456876b-0849-0410-b77d-98878d47e9d5