logproto: Fix the framed server free method

The LogProtoFramedServer free method did not call the free_method of its
super class, which resulted in memory & FD leaks when using a syslog()
source. This patch corrects that.

Reported-by: Andras Mitzki <micek@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>