igb: check memory allocation failure

Check memory allocation failures and return -ENOMEM in such cases, as
already done for other memory allocations in this function.

This avoids NULL pointers dereference.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Acked-by: PJ Waskiewicz <peter.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>