pgsql: Fix crash in json_to_record().

Fix crash in json_to_record().

json_to_record() depends on get_call_result_type() for the tuple
descriptor of the record that should be returned, but in some cases
that cannot be determined. Add a guard to check if the tuple
descriptor has been properly resolved, similar to other callers of
get_call_result_type().

Also add guard for two other callers of get_call_result_type() in
jsonfuncs.c. Although json_to_record() is the only actual bug, it's a
good idea to follow convention.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/486ea0b19e08c10ff53e36e46209a928df048281

Modified Files
--------------
src/backend/utils/adt/jsonfuncs.c |   20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)