BUG/MINOR: stats-json: Define JSON_INT_MAX as a signed integer

BUG/MINOR: stats-json: Define JSON_INT_MAX as a signed integer

A JSON integer is defined in the range [-(2**53)+1, (2**53)-1]. Macro are used
to define the minimum and the maximum value, The minimum one is defined using
the maximum one. So JSON_INT_MAX must be defined as a signed integer value to
avoid wrong cast of JSON_INT_MIN.

It was reported by Coverity in #2841: CID 1587769.

This patch could be backported to all stable versions.

(cherry picked from commit d48b5add889db1bf2f0fae4721abb46413303d33)
Signed-off-by: Willy Tarreau <w@1wt.eu>