Buffer name inconsistently used a cstring/vec (VPP-901)

Buffer name inconsistently used a cstring/vec (VPP-901)

Spotted in the output of CLI command "show buffers", the name field
sometimes had trailing garbage, the hall sign of a string not being
terminated. In this case it was being inconsistently used as a cstring
or a vec.

- CLI printf needs %v to print the vec srring
- vlib_buffer_create_free_list_helper tried to use
  clib_mem_is_heap_object() to detect a vec object, wheras it should
  use clib_mem_is_vec()

Change-Id: Ib8b242a0c5a18924b8af7e8e1432784eebcf572c
Signed-off-by: Chris Luke <chrisy@flirble.org>