Make json{b}_populate_recordset() use the right tuple descriptor.

json{b}_populate_recordset() used the tuple descriptor created from the
query-level AS clause without worrying about whether it matched the actual
input record type.  If it didn't, that would usually result in a crash,
though disclosure of server memory contents seems possible as well, for a
skilled attacker capable of issuing crafted SQL commands.  Instead, use
the query-supplied descriptor only when there is no input tuple to look at,
and otherwise get a tuple descriptor based on the input tuple's own type
marking.  The core code will detect any type mismatch in the latter case.

Michael Paquier and Tom Lane, per a report from David Rowley.
Back-patch to 9.3 where this functionality was introduced.

Security: CVE-2017-15098

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