syslog-ng: Fix memory leak when udp connection is used [ LIN7-1379 ]

When udp connection is used, there are several memory leaks happen
after run a long time.

Signed-off-by: Xufeng Zhang <xufeng.zhang@windriver.com>
Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>