ipfilter: Fix ip_nat memory leak and use-after-free

Unfortunately the wrong elemet is freed, also resulting in use-after-free.

PR: 255859
Submitted by: lylgood@foxmail.com
Reported by: lylgood@foxmail.com

(cherry picked from commit 323a4e2c4e285e6f8eee8db3fe2cb7490a734da0)