Reset frame length after dispatch when recycling frames

Fixes a gross bug when enqueuing x->x->error-drop or similar

Change-Id: Ie1dc10371de4a265f5131b67cab7a62ed460a1fc
Signed-off-by: Dave Barach <dave@barachs.net>

vlib/vlib/main.c

@@ -1146,6 +1146,7 @@ dispatch_pending_node (vlib_main_t * vm,
       nf = vec_elt_at_index (nm->next_frames, p->next_frame_index);
       nf->frame_index = restore_frame_index;
       nf->flags |= VLIB_FRAME_IS_ALLOCATED;
+      f->n_vectors = 0;
     }
 
   if (f->flags & VLIB_FRAME_FREE_AFTER_DISPATCH)