misc: check return values from vlib_buffer_copy(...)

vlib_buffer_copy(...) returns NULL if the system is temporarily out of
buffers.

This is NOT correct. Please don't be this person:

   c0 = vlib_buffer_copy (vm, p0);
   ci0 = vlib_get_buffer_index (vm, c0);

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ic25ef58965871ea5d2b40904df9506803f69e47e