Fix a long-latent bi=0 bug in vlib_buffer_add_data

Fix a long-latent bi=0 bug in vlib_buffer_add_data

Change vlib_buffer_add_data() so it interprets ~0 to mean please
allocate a new buffer, instead of 0. Fixed a couple of calls to pass
~0 instead of 0.

Zero has always been a valid buffer index, we never happened to
actually use it until recent buffer allocator changes.

The presenting symptom: ASSERT failure when running "make
TEST=test_mpls test-debug"

Change-Id: Ic909913c1d464b3434d6d47e0c58f978806854d5
Signed-off-by: Dave Barach <dave@barachs.net>