Fix Assert failure in ExpandColumnRefStar --- what I thought was a can't

Fix Assert failure in ExpandColumnRefStar --- what I thought was a can't
happen condition can happen given incorrect input.  The real problem is that
gram.y should try harder to distinguish * from "*" --- the latter is a legal
column name per spec, and someday we ought to treat it that way.  However
fixing that is too invasive for a back-patch, and it's too late for the 8.3
cycle too.  So just reduce the Assert to a plain elog for now.  Per report
from NikhilS.