JSON_KEYS with a path returns keys out of a document that is broken outside the selected sub-object

JSON_KEYS is documented to raise an error when the document is invalid.
Given a path, it reads only as far as the sub-object the path selects,
so a break anywhere outside that sub-object is never met.

SELECT JSON_KEYS('{"a":{"z":1} "b":2}', '$.a') AS v;

v

["z"]

No warning. Without the path, the same bytes are refused:

SELECT JSON_KEYS('{"a":1 "b":2}') AS v;

v

NULL

Warnings:

Warning  4038  Syntax error in JSON text in argument 1

              to function 'json_keys' at position 8

So the presence of a path argument decides whether an invalid document
is diagnosed.

How to repeat

SELECT JSON_KEYS('{"a":{"z":1} "b":2}', '$.a')  AS v;   -- ["z"], silent

SELECT JSON_KEYS('{"a":{"z":1},"b":2,}', '$.a') AS v;   -- ["z"], silent

SELECT JSON_KEYS('[{"z":1} 2]', '$[0]')         AS v;   -- ["z"], silent

Root cause

The walk stops at the end of the selected sub-object, so a break past
that end is never reached and the scanner never reports one. A break
inside the selected sub-object is met and does refuse the answer.

Contract, `scripts/fill_help_tables.sql:444` (topic 370): *"An error
will occur if JSON document is invalid, the path is invalid or if the
path contains a or * wildcard."* The wildcard half of that sentence is
honoured, as Warning 4044.

Affected versions

JSON_KEYS was added in 10.2.3.

Verified on 10.11 only (10.11.19-MariaDB-debug, commit 1dab253482d).
Other branches were not tested.

Note

Found while auditing the JSON functions against their documented
contracts. None. Every two-argument JSON_KEYS in the suite
has a well-formed outer document; func_json_invalid.test:74 covers
only the one-argument form.