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

Log Message:
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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.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/parser:
        parse_target.c (r1.149 -> r1.149.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_target.c?r1=1.149&r2=1.149.2.1)