Incorrect matching of sql/json PASSING variable names

Hi,

Alvaro reported off-list that the following should really fail,
because the jsonpath expression refers to a PASSING variable that
doesn't exist:

select json_query('"1"', jsonpath '$xy' passing 2 AS xyz);
json_query
------------
2
(1 row)

This works because of a bug in GetJsonPathVar() whereby it allows a
jsonpath expression to reference any prefix of the PASSING variable
names.

Attached is a patch to fix that.

Thanks Alvaro for the report.

--
Thanks, Amit Langote