BUG/MINOR: http-act: fix a double free of the map reference on a parsing error

parse_http_set_map() extracts the map/acl file name into <rule->arg.map.ref>,
then parses one or two log-format strings. On a parsing failure it releases the
reference before reporting the error:

	if (!parse_logformat_string(args[cur_arg], px, &rule->arg.map.key, ...)) {
		free(rule->arg.map.ref);
		return ACT_RET_PRS_ERR;
	}

but <rule->release_ptr> has already been set to release_http_map(), which starts
with "free(rule->arg.map.ref)". Since the caller calls free_act_rule() upon
ACT_RET_PRS_ERR, the same pointer is freed twice. Both error paths of the
function (the key and the value patterns) are affected, and they cover the
"add-acl", "del-acl", "set-map" and "del-map" actions.

Reproduced with:

	http-request set-map(/tmp/m.map) %[nosuchfetch] somevalue

which reports "free(): double free detected in tcache 2" and aborts, so
"haproxy -c" dies instead of reporting the configuration error.

This only happens while parsing an invalid configuration, so there is no
security impact, but a configuration checker must not crash.

Let's use ha_free() so the pointer is reset and release_http_map() becomes a
no-op for it.

This should be backported to all supported versions.