BUG/MINOR: http-rules: fix release of a failed "set-cookie-fmt" redirect rule

<redirect_rule.cookie> is a union holding either an ist (set-cookie,
clear-cookie) or a log-format expression (set-cookie-fmt), and
http_free_redirect_rule() picks the member to release from the
REDIRECT_FLAG_COOKIE_FMT flag:

	if ((rdr->flags & REDIRECT_FLAG_COOKIE_FMT))
		lf_expr_deinit(&rdr->cookie.fmt);
	else
		istfree(&rdr->cookie.str);

But http_parse_redirect_rule() accumulates the flags in a local variable and
only assigns them with "rule->flags = flags" at the very end, once
everything succeeded. So when the log-format string of a "set-cookie-fmt"
fails to parse, the "goto err" runs http_free_redirect_rule() on a rule
whose ->flags is still 0, and istfree() is called on the <str> member while
<fmt> is the live one, leading the process to crash.

Let's init the rule members (code, type and flags) as soon as possible, so
after the rule allocation. And REDIRECT_FLAG_COOKIE_FMT flag is not set
directly on the rule's flags, before the log-format string parsing.

This only happens while parsing an invalid configuration, hence no security
impact, but a configuration checker must report errors rather than abort.

This should be backported to all versions having "set-cookie-fmt", so 2.9 and
above.