[BUG] stats: session rate limit gets garbaged in the stats

[BUG] stats: session rate limit gets garbaged in the stats

The statistics page (the HTML one) displays a garbage value on frontends using
"rate-limit session" in HTTP mode.

This is due to the usage of the same buffer for the macros converting the max
session rate and the limit.

Steps to reproduce :
Configuration file example  :
listen bug :80
  mode http
  rate-limit sessions
  stats enable

Then start refreshing the statistics page.

This bug was introduced just before the release of haproxy 1.4.0.

(was commit 6cfaf9e91969c87a9eab1d58a15d2d0a3f346c9b in 1.4)