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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REL7_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=42c387166d4cd3e7151c88692a157bfe9a621a4d

Modified Files
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src/backend/parser/parse_func.c |   80 +++++++++++++++++++++++++++------------
1 files changed, 56 insertions(+), 24 deletions(-)