Fix NULL timeout dereference in ff_lro_flush

Pass a zero timeval to tcp_lro_flush_inactive instead of NULL to avoid
a segfault when lro_active is non-empty (tcp_lro.c:633 dereferences the
timeout pointer unconditionally after the empty-list early return).

Author: jfb8856606

lib/ff_veth.c

@@ -543,7 +543,8 @@ ff_lro_rx(void *lro, void *m)
 void
 ff_lro_flush(void *lro)
 {
-    tcp_lro_flush_inactive(lro, NULL);
+    struct timeval tv = {0, 0};
+    tcp_lro_flush_inactive(lro, &tv);
 }
 
 void