Fix typo in trace trajectory init

plugin won't compile if VLIB_BUFFER_TRACE_TRAJECTORY is set. The
quad loop was OK, bug in single loop only.

Change-Id: I11262bf6a223680eeac7ed23238ceda425a02c1b
Signed-off-by: Dave Barach <dave@barachs.net>
diff --git a/src/plugins/avf/input.c b/src/plugins/avf/input.c
index efec723..d11c3cf 100644
--- a/src/plugins/avf/input.c
+++ b/src/plugins/avf/input.c
@@ -304,7 +304,7 @@
       clib_memcpy (vnet_buffer (b[0])->sw_if_index,
 		   vnet_buffer (bt)->sw_if_index, 2 * sizeof (u32));
 
-      VLIB_BUFFER_TRACE_TRAJECTORY_INIT (b0);
+      VLIB_BUFFER_TRACE_TRAJECTORY_INIT (b[0]);
 
       /* next */
       rxve += 1;