BUG/MEDIUM: log: ensure that unique_id is properly initialized

Last memory poisonning patch immediately made this issue appear.
The unique_id field is released but not properly initialized. The
feature was introduced very recently, no backport is needed.
diff --git a/src/peers.c b/src/peers.c
index 182d71d..42018e1 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1204,6 +1204,7 @@
 	s->srv_error = default_srv_error;
 
 	s->uniq_id = 0;
+	s->unique_id = NULL;
 
 	txn = &s->txn;
 	/* Those variables will be checked and freed if non-NULL in