logreader: Memory leak fix.

The log_reader_set_peer_addr() function is called with every reload. If
peer_addr is set, it will be ref'd again.

Configuration to reproduce:
 1) syslog-ng creates a TCP source
 2) connect to it with netcat (and don't close the connection)
 3) reload syslog-ng
 4) send SIGTERM to syslog-ng

Signed-off-by: Tibor Benke <ihrwein@gmail.com>