BUG/MAJOR: vars: Fix bogus free() during deinit() for http-request rules

BUG/MAJOR: vars: Fix bogus free() during deinit() for http-request rules

We cannot simply `release_sample_expr(rule->arg.vars.expr)` for a
`struct act_rule`, because `rule->arg` is a union that might not
contain valid `vars`. This leads to a crash on a configuration using
`http-request redirect` and possibly others:

    frontend http
    	mode http
    	bind 127.0.0.1:80
    	http-request redirect scheme https

Instead a `struct act_rule` has a `release_ptr` that must be used
to properly free any additional storage allocated.

This patch fixes a regression in commit ff78fcdd7f15c8626c7e70add7a935221ee2920c.
It must be backported to whereever that patch is backported.

It has be verified that the configuration above no longer crashes.
It has also been verified that the configuration in ff78fcdd7f15c8626c7e70add7a935221ee2920c
does not leak.