udp: fix coverity warning

Type: fix

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I8efde7a4be9b04eae7e30b153de5edab7854496b
diff --git a/src/vnet/udp/udp_input.c b/src/vnet/udp/udp_input.c
index 0a4af60..95595c3 100644
--- a/src/vnet/udp/udp_input.c
+++ b/src/vnet/udp/udp_input.c
@@ -98,7 +98,7 @@
   t = vlib_add_trace (vm, node, b, sizeof (*t));
   t->connection = s ? s->connection_index : ~0;
   t->disposition = error0;
-  t->thread_index = s->thread_index;
+  t->thread_index = s ? s->thread_index : vm->thread_index;
 }
 
 static udp_connection_t *