cdp/lldp: punt for no buffer (VPP-997)

cdp/lldp: punt for no buffer (VPP-997)

When making a call to vlib_packet_template_get_packet(), it
is possible to get back a NULL if the system runs out of buffer.
This can happen when there is buffer leaks. But don't crash
just because we run out of buffers, just punt.

Change-Id: Ie90ea41f3dda6e583d48959cbd18ff124158d7f8
Signed-off-by: Steven <sluong@cisco.com>
(cherry picked from commit 0ff5c563d5048991dbd02a3892dccde8305a7e30)
(cherry picked from commit 1808f3c00a7bcdea7f0c004ef0613db2156c2065)