pgsql: Improved version of patch to protect pg_get_expr() against

Log Message:
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Improved version of patch to protect pg_get_expr() against misuse:
look through join alias Vars to avoid breaking join queries, and
move the test to someplace where it will catch more possible ways
of calling a function.  We still ought to throw away the whole thing
in favor of a data-type-based solution, but that's not feasible in
the back branches.

Completion of back-port of my patch of yesterday.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/parser:
        parse_expr.c (r1.185.2.3 -> r1.185.2.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.185.2.3&r2=1.185.2.4)
        parse_func.c (r1.182.2.1 -> r1.182.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.182.2.1&r2=1.182.2.2)
        parse_oper.c (r1.82.2.1 -> r1.82.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.82.2.1&r2=1.82.2.2)
    pgsql/src/include/parser:
        parse_func.h (r1.53 -> r1.53.6.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.53&r2=1.53.6.1)