BUG/MEDIUM: regex: fix risk of buffer overrun in exp_replace()

Currently exp_replace() (which is used in reqrep/reqirep) is
vulnerable to a buffer overrun. I have been able to reproduce it using
the attached configuration file and issuing the following command:

  wget -O - -S -q http://localhost:8000/`perl -e 'print "a"x4000'`/cookie.php

Str was being checked only in in while (str) and it was possible to
read past that when more than one character was being accessed in the
loop.

WT:
   Note that this bug is only marked MEDIUM because configurations
   capable of triggering this bug are very unlikely to exist at all due
   to the fact that most rewrites consist in static string additions
   that largely fit into the reserved area (8kB by default).

   This fix should also be backported to 1.4 and possibly even 1.3 since
   it seems to have been present since 1.1 or so.

Config:
-------

  global
        maxconn         500
        stats socket /tmp/haproxy.sock mode 600

  defaults
        timeout client      1000
        timeout connect      5000
        timeout server      5000
        retries         1
        option redispatch

  listen stats
        bind :8080
        mode http
        stats enable
        stats uri /stats
        stats show-legends

  listen  tcp_1
        bind :8000
        mode            http
        maxconn 400
        balance roundrobin
        reqrep ^([^\ :]*)\ /(.*)/(.*)\.php(.*) \1\ /\3.php?arg=\2\2\2\2\2\2\2\2\2\2\2\2\2\4
        server  srv1 127.0.0.1:9000 check port 9000 inter 1000 fall 1
        server  srv2 127.0.0.1:9001 check port 9001 inter 1000 fall 1