i40e/i40evf: fix bug when skb allocation fails

If the skb allocation fails we should not continue using the skb
pointer.  Breaking out at the point of failure means that at the next
RX interrupt the driver will try the allocation again.

Change-ID: Iefaad69856ced7418bfd92afe55322676341f82e
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>