vlib:init free list buffers vec

fixes a problem seen when dhcp proxy uses vlib_buffer_chain_linearize
function which tries to vlib_buffer_alloc zero buffers - which succeeds
if the buffers vector is initialized but otherwise crashes when trying to
update the _vec_len in vlib_buffer_alloc_from_free_list
solved by initializing the free_list buffers vec

Change-Id: I1186d7aac05e54864d72f3f144e1bef8064f8efa
Signed-off-by: Eyal Bari <ebari@cisco.com>