af_packet driver failure to check VLIB_BUFFER_NEXT_PRESENT flag

af_packet driver failure to check VLIB_BUFFER_NEXT_PRESENT flag

af_packet driver must check that VLIB_BUFFER_NEXT_PRESENT flag is set
when walking vlib_buffer_t next_buffer chain on transmit.
On buffer allocation:
- next_buffer is not and may contain a stale invalid value that
  should be ignored if not overwritten by a valid value.
- VLIB_BUFFER_NEXT_PRESENT flag is cleared and only set
  if a valid value is written to next_buffer.

Change-Id: Iebf76ce8eea24a0d63c7bf749e672d6a232c80e7
Signed-off-by: Jim Gibson <gibson+fdio@cisco.com>