Patch to fix warnings when searching for invalid username

If you search for "user:NOSUCHUSER" you'll get a variety of PHP warnings from trying to implode a non-array.
This trivial patch resolves it, but still leaves a problem that it puts up 3 "user missing" error messages. It's
an improvement over the prior problem.

Patch against 9/15/2005 CVS:
http://www.menalto.com/.outgoing/drupal/trip_search.module.patch1