igb: fix null pointer dereference seen with fiber NICs

With a fiber or serdes NIC installed the driver was causing a null pointer
dereference on driver unload.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>