Fix crash in json{b}_populate_recordset() and json{b}_to_recordset().

As of commit 37a795a60, populate_recordset_worker() tried to pass back
(as rsi.setDesc) a tupdesc that it also had cached in its fn_extra.
But the core executor would free the passed-back tupdesc, risking a
crash if the function were called again in the same query.  The safest
and least invasive way to fix that is to make an extra tupdesc copy
to pass back.

While at it, I failed to resist the temptation to get rid of unnecessary
get_fn_expr_argtype() calls here and in populate_record_worker().

Per report from Dmitry Dolgov; thanks to Michael Paquier and
Andrew Gierth for investigation and discussion.

Discussion: https://postgr.es/m/CA+q6zcWzN9ztCfR47ZwgTr1KLnuO6BAY6FurxXhovP4hxr+yOQ@mail.gmail.com

src/test/regress/expected/json.out

@@ -1841,6 +1841,16 @@ SELECT json_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
  (0,1)
 (1 row)
 
+SELECT i, json_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
+FROM (VALUES (1),(2)) v(i);
+ i | json_populate_recordset 
+---+-------------------------
+ 1 | (42,50)
+ 1 | (1,43)
+ 2 | (42,50)
+ 2 | (2,43)
+(4 rows)
+
 -- composite domain
 SELECT json_populate_recordset(null::j_ordered_pair, '[{"x": 0, "y": 1}]');
  json_populate_recordset 

src/test/regress/expected/jsonb.out

@@ -2523,6 +2523,16 @@ SELECT jsonb_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
  (0,1)
 (1 row)
 
+SELECT i, jsonb_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
+FROM (VALUES (1),(2)) v(i);
+ i | jsonb_populate_recordset 
+---+--------------------------
+ 1 | (42,50)
+ 1 | (1,43)
+ 2 | (42,50)
+ 2 | (2,43)
+(4 rows)
+
 -- composite domain
 SELECT jsonb_populate_recordset(null::jb_ordered_pair, '[{"x": 0, "y": 1}]');
  jsonb_populate_recordset 

src/test/regress/sql/json.sql

@@ -547,6 +547,8 @@ select * from json_populate_recordset(row('def',99,null)::jpop,'[{"a":[100,200,3
 -- anonymous record type
 SELECT json_populate_recordset(null::record, '[{"x": 0, "y": 1}]');
 SELECT json_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
+SELECT i, json_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
+FROM (VALUES (1),(2)) v(i);
 
 -- composite domain
 SELECT json_populate_recordset(null::j_ordered_pair, '[{"x": 0, "y": 1}]');

src/test/regress/sql/jsonb.sql

@@ -663,6 +663,8 @@ SELECT * FROM jsonb_populate_recordset(row('def',99,NULL)::jbpop,'[{"a":[100,200
 -- anonymous record type
 SELECT jsonb_populate_recordset(null::record, '[{"x": 0, "y": 1}]');
 SELECT jsonb_populate_recordset(row(1,2), '[{"f1": 0, "f2": 1}]');
+SELECT i, jsonb_populate_recordset(row(i,50), '[{"f1":"42"},{"f2":"43"}]')
+FROM (VALUES (1),(2)) v(i);
 
 -- composite domain
 SELECT jsonb_populate_recordset(null::jb_ordered_pair, '[{"x": 0, "y": 1}]');