MINOR: stream: Use stream_generate_unique_id

This patch replaces the ad-hoc generation of stream's `unique_id` values
by calls to `stream_generate_unique_id`.
diff --git a/src/log.c b/src/log.c
index 60b1a5a..b46605b 100644
--- a/src/log.c
+++ b/src/log.c
@@ -2983,8 +2983,7 @@
 
 	/* if unique-id was not generated */
 	if (!s->unique_id && !LIST_ISEMPTY(&sess->fe->format_unique_id)) {
-		if ((s->unique_id = pool_alloc(pool_head_uniqueid)) != NULL)
-			build_logline(s, s->unique_id, UNIQUEID_LEN, &sess->fe->format_unique_id);
+		stream_generate_unique_id(s, &sess->fe->format_unique_id);
 	}
 
 	if (!LIST_ISEMPTY(&sess->fe->logformat_sd)) {