as this node is refactored in new style with
vlib_buffer_enqueue_to_next, we have to check if the 'count'
is greater than 0. otherise, the next_index would be invalid
then lead to a crash
Change-Id: If7c323b59c02b5c16bd9d77b65c946512cc972c1
Signed-off-by: Kingwel Xie <kingwel.xie@ericsson.com>