[syslog-ng] segfaults when a filter does not exist

[Kenneth Kassing]

I have recently produced a segfault in code version 1.6.9 when I use a 
filter which is not defined.

In the example configuration snippet below, I left the filter 'f_test' 
undefined.  Under normal conditions you will be unaware the process 
fails because it is backgrounded.  If started with the '-F' argument, it 
reports a segfault on the command line.

filter f_syslog { not facility(auth, authpriv) and not filter(f_test); };