[BUG] don't set an expiration date directly from now_ms

now_ms can be zero, don't set ->analyse_exp directly from it, we
must use tick_add() instead.
diff --git a/src/proto_http.c b/src/proto_http.c
index 8ab4199..359488b 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2313,7 +2313,7 @@
 		req->analysers |= AN_REQ_HTTP_TARPIT;
 		req->analyse_exp = tick_add_ifset(now_ms,  s->be->timeout.tarpit);
 		if (!req->analyse_exp)
-			req->analyse_exp = now_ms;
+			req->analyse_exp = tick_add(now_ms, 0);
 	}
 
 	/* OK let's go on with the BODY now */