pgsql: Fix Assert failure in ExpandColumnRefStar --- what I thought was

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_target.c (r1.155 -> r1.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_target.c?r1=1.155&r2=1.156)