pgsql: Put back plan-time check for trying to apply SELECT FOR

Log Message:
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Put back plan-time check for trying to apply SELECT FOR UPDATE/SHARE
to a relation on the nullable side of an outer join. I had removed
this during the outer join planning rewrite a few months ago ... I think
I intended to put it somewhere else, but forgot ...

Modified Files:
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pgsql/src/backend/optimizer/plan:
initsplan.c (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/initsplan.c.diff?r1=1.120&r2=1.121)