BUG/MINOR: proxy: fix server_id_hdr_name leak on deinit()

proxy server_id_hdr_name member (used for "http-send-name-header" option)
wasn't cleaned up in free_proxy(), resulting in small memory leak if
"http-send-name-header" was used on a regular or default proxy.

This may be backported to all stable versions.

src/proxy.c

@@ -218,6 +218,7 @@ void free_proxy(struct proxy *p)
 	free(p->server_state_file_name);
 	free(p->capture_name);
 	istfree(&p->monitor_uri);
+	istfree(&p->server_id_hdr_name);
 	free(p->rdp_cookie_name);
 	free(p->invalid_rep);
 	free(p->invalid_req);