Silence compiler warnings about possibly unset variables.

They are in fact set in every case where they are needed, but the
compiler doesn't know that.

Per gripe from Tom Lane.

src/backend/utils/adt/jsonfuncs.c

@@ -1283,8 +1283,8 @@ populate_record_worker(PG_FUNCTION_ARGS, bool have_record_arg)
 	bool		use_json_as_text;
 	HTAB	   *json_hash;
 	HeapTupleHeader rec = NULL;
-	Oid			tupType;
+	Oid			tupType = InvalidOid;
-	int32		tupTypmod;
+	int32		tupTypmod = -1;
 	TupleDesc	tupdesc;
 	HeapTupleData tuple;
 	HeapTuple	rettuple;