BUG/MINOR: http: Memory leak of http redirect rules' format string

BUG/MINOR: http: Memory leak of http redirect rules' format string

When the configuration contains such a line:
    http-request redirect location /
a "struct logformat_node" object is created and it contains an "arg"
member which gets alloc'ed as well in which we copy the new location
(see add_to_logformat_list). This internal arg pointer was not freed in
the dedicated release_http_redir release function.
Likewise, the expression pointer was not released as well.

This patch can be backported to all stable branches. It should apply
as-is all the way to 2.2 but it won't on 2.0 because release_http_redir
did not exist yet.