json: even stricter trailing garbage detection

json: even stricter trailing garbage detection

Since older yajl ignores trailing garbage, a client can cause
problems by intentionally ending the wrapper array early. Since
we already track nesting, it's not too much harder to reject
invalid nesting pops.

* src/util/virjson. (_virJSONParser): Add field.
(virJSONValueFromString): Set witness.
(virJSONParserHandleEndArray): Use it to catch abuse.
* tests/jsontest.c (mymain): Test it.

Signed-off-by: Eric Blake <eblake@redhat.com>