BUG/MINOR: proxy: fix logformat expression leak in use_backend rules

When support for dynamic names was added for use_backend rules in
702d44f2f ("MEDIUM: proxy: support use_backend with dynamic names"), the
sample expression resulting from parse_logformat_string() was only freed
for non dynamic rules (when the expression resolved to a simple string
node). But for complex expressions (ie: multiple nodes), rule->dynamic
was set but the expression was never released, resulting in a small
memory leak when freeing the parent proxy.

To fix the issue, in free_proxy(), we free the switching rule expression
if the switching rule is dynamic.

This should be backported to every stable versions.
[ada: prior to 2.9, free_logformat_list() helper did not exist: we may
 use the same manual sample expr freeing logic as in server_rules pruning
 right above it]

src/proxy.c

@@ -263,6 +263,8 @@ void free_proxy(struct proxy *p)
 	list_for_each_entry_safe(rule, ruleb, &p->switching_rules, list) {
 		LIST_DELETE(&rule->list);
 		free_acl_cond(rule->cond);
+		if (rule->dynamic)
+			free_logformat_list(&rule->be.expr);
 		free(rule->file);
 		free(rule);
 	}