BUG/MEDIUM: dns: fix memory leak of sockaddr in dns_session_init() error path

In dns_session_init(), sockaddr_alloc() allocates 'addr' from the sockaddr
pool, but on failure of appctx_finalize_startup() we jump to the error label
without calling sockaddr_free(&addr), leaking the allocation. Let's add the
missing sockaddr_free() on the error branch.

This must be backported to 2.6.

src/dns.c

@@ -913,6 +913,7 @@ static int dns_session_init(struct appctx *appctx)
 	return 0;
 
   error:
+	sockaddr_free(&addr);
 	return -1;
 }