BUG/MEDIUM: sink: invalid server list in sink_new_from_logsrv()

BUG/MEDIUM: sink: invalid server list in sink_new_from_logsrv()

forward proxy server list created from sink_new_from_logsrv() is invalid

Indeed, srv->next is literally assigned to itself. This did not cause
issues during syslog handling because the sft was properly set, but it
will cause the free_proxy(sink->forward_px) at deinit to go wild since
free_proxy() will try to iterate through the proxy srv list to free
ressources, but because of the improper list initialization, double-free
and infinite-loop will occur.

This bug was revealed by 9b1d15f53a ("BUG/MINOR: sink: free forward_px on deinit()")

It must be backported as far as 2.4.