[#MDEV-12311] Insufficient check for argument validity in JSON functions

Example is for JSON_LENGTH, probably other functions are similarly affected, please keep them consistent.

JSON_LENGTH does some check on argument validity:

10.2 (OK)
MariaDB [test]> select json_length('{{');
+-------------------+
\| json_length('{{') \|
+-------------------+
\| NULL \|
+-------------------+
1 row in set, 1 warning (0.00 sec)

MariaDB [test]> show warnings;
+---------+------+---------------------------------------------------------------------------------+
\| Level \| Code \| Message \|
+---------+------+---------------------------------------------------------------------------------+
\| Warning \| 4038 \| Syntax error in JSON text in argument 1 to function 'json_length' at position 2 \|
+---------+------+---------------------------------------------------------------------------------+
1 row in set (0.00 sec)

But it does not notice obvious problems in the argument:

10.2 (not OK)
MariaDB [test]> select json_length('{}{');
+--------------------+
\| json_length('{}{') \|
+--------------------+
\| 0 \|
+--------------------+
1 row in set (0.00 sec)

MySQL provides decent diagnostics:

MySQL 5.7
MySQL [test]> select json_length('{}{');
ERROR 3141 (22032): Invalid JSON text in argument 1 to function json_length: "The document root must not follow by other values." at position 2.

[MDEV-12311] Insufficient check for argument validity in JSON functions Created: 2017-03-20 Updated: 2017-10-05 Resolved: 2017-10-05
Status:	Closed
Project:	MariaDB Server
Component/s:	JSON
Affects Version/s:	10.2
Fix Version/s:	10.2.10