BUG/CRITICAL: mjson: fix possible DoS when parsing numbers

BUG/CRITICAL: mjson: fix possible DoS when parsing numbers

Mjson comes with its own strtod() implementation for portability
reasons and probably also because many generic strtod() versions as
provided by operating systems do not focus on resource preservation
and may call malloc(), which is not welcome in a parser.

The strtod() implementation used here apparently originally comes from
https://gist.github.com/mattn/1890186 and seems to have purposely
omitted a few parts that were considered as not needed in this context
(e.g. skipping white spaces, or setting errno). But when subject to the
relevant test cases of the designated file above, the current function
provides the same results.

The aforementioned implementation uses pow() to calculate exponents,
but mjson authors visibly preferred not to introduce a libm dependency
and replaced it with an iterative loop in O(exp) time. The problem is
that the exponent is not bounded and that this loop can take a huge
amount of time. There's even an issue already opened on mjson about
this: https://github.com/cesanta/mjson/issues/59. In the case of
haproxy, fortunately, the watchdog will quickly stop a runaway process
but this remains a possible denial of service.

A first approach would consist in reintroducing pow() like in the
original implementation, but if haproxy is built without Lua nor
51Degrees, -lm is not used so this will not work everywhere.

Anyway here we're dealing with integer exponents, so an easy alternate
approach consists in simply using shifts and squares, to compute the
exponent in O(log(exp)) time. Not only it doesn't introduce any new
dependency, but it turns out to be even faster than the generic pow()
(85k req/s per core vs 83.5k on the same machine).

This must be backported as far as 2.4, where mjson was introduced.

Many thanks to Oula Kivalo for reporting this issue.

Gbp-Pq: Name 0001-BUG-CRITICAL-mjson-fix-possible-DoS-when-parsing-num.patch