ipg: use NULL, not zero, for pointers

Fixes a sparse warning in a code block that's hidden under JUMBO_FRAME #ifdef.

Tested-by: Andrew Savchenko <Bircoph@list.ru>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>