dissector_eth: sparse: fix sparse warning

This fixes the following sparse warnings:

 dissector_eth.c:43:16: warning: Using plain integer as NULL pointer
 dissector_eth.c:48:16: warning: Using plain integer as NULL pointer
 dissector_eth.c:53:16: warning: Using plain integer as NULL pointer

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>