pgsql: Further fixes to the pg_get_expr() security fix in back branches

Further fixes to the pg_get_expr() security fix in back branches.

It now emerges that the JDBC driver expects to be able to use pg_get_expr()
on an output of a sub-SELECT.  So extend the check logic to be able to recurse
into a sub-SELECT to see if the argument is ultimately coming from an
appropriate column.  Per report from Thomas Kellerer.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=78b0a0d8c026d20788b584266c2aef5f5add16b2

Modified Files
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src/backend/parser/parse_func.c |   84 +++++++++++++++++++++++++++------------
1 files changed, 58 insertions(+), 26 deletions(-)