Bug in searching path in jsonb_set when walking through JSONB array

Hello, Hackers!

While I was reviewed a patch with "json_insert" function I found a bug
which wasn't connected with the patch and reproduced at master.

It claims about non-integer whereas input values are obvious integers
and in an allowed range.
More testing lead to understanding it appears when numbers length are
multiplier of 4:

postgres=# select jsonb_set('{"a":[[],1,2,3],"b":[]}', '{"a", 9999}', '"4"');
ERROR: path element at the position 2 is not an integer

postgres=# select jsonb_set('{"a":[[],1,2,3],"b":[]}', '{"b", 1000}', '"4"');
ERROR: path element at the position 2 is not an integer

postgres=# select jsonb_set('{"a":[[],1,2,3],"b":[]}', '{"a", -999}', '"4"');
ERROR: path element at the position 2 is not an integer

postgres=# select jsonb_set('{"a":[[],1,2,3],"b":[]}', '{"a",
10009999}', '"4"');
ERROR: path element at the position 2 is not an integer

Close values are ok:
postgres=# select jsonb_set('{"a":[[],1,2,3]}', '{"a", 0, 999}', '"4"');
jsonb_set
-------------------------
{"a": [["4"], 1, 2, 3]}
(1 row)

postgres=# select jsonb_set('{"a":[[],1,2,3]}', '{"a", 0, 10000}', '"4"');
jsonb_set
-------------------------
{"a": [["4"], 1, 2, 3]}
(1 row)

Research lead to setPathArray where a string which is got via
VARDATA_ANY but is passed to strtol which expects cstring.

In case of string number length is not a multiplier of 4 rest bytes
are padding by '\0', when length is a multiplier of 4 there is no
padding, just garbage after the last digit of the value.

Proposed patch in an attachment fixes it.

There is a magic number "20" as a length of an array for copying key
from a path before passing it to strtol. It is a maximal length of a
value which can be parsed by the function. I could not find a proper
constant for it. Also I found similar direct value in the code (e.g.
in numeric.c).

I've added a comment, I hope it is enough for it.

--
Best regards,
Vitaly Burovoy