ldpd: fix heap-use-after-free at exit

ldpd: fix heap-use-after-free at exit

This problems happens because, in this port, whenever the child
processes want to log something they send a message to the parent. But
in the shutdown functions the first thing we do is to close the pipes
to the parent process. With that said, add some protections to prevent
the child processes from trying to use a closed pipe and just ignore
their log messages during shutdown. In the future we need to share
the logging configuration with the child processes so they can send
log messages on their own.

While here, remove some unnecessary calls to msgbuf_write() in
ldpe_shutdown().

Fixes #1253.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>