MINOR: log: make sess_build_logline() not dereference a NULL stream for txn

If the stream is NULL, the txn is NULL as well. This condition is already
handled everywhere else.
diff --git a/src/log.c b/src/log.c
index 2613940..eaa8cf3 100644
--- a/src/log.c
+++ b/src/log.c
@@ -1572,7 +1572,7 @@
 {
 	struct proxy *fe = sess->fe;
 	struct proxy *be = s ? s->be : fe;
-	struct http_txn *txn = s->txn;
+	struct http_txn *txn = s ? s->txn : NULL;
 	struct buffer chunk;
 	char *uri;
 	char *spc;