Handle buffer alloc failure in vlib_buffer_add_data

It's not OK to crash due to a transient buffer allocation failure.

Return 1 if the requested operation failed, otherwise 0.

Buffer index parameter change to a value-result, so the caller can
differentiate between partial and complete allocation failure: callers
which request an initial allocation (inbound bi = ~0) need to check
the (out) value to decide whether or not to call vlib_buffer_free(...).

Change-Id: I03029d7f2714c17dca4630dfd95a1eb578b68384
Signed-off-by: Dave Barach <dave@barachs.net>