i40e/i40evf: Fix use after free in Rx cleanup path

i40e/i40evf: Fix use after free in Rx cleanup path

We need to reset skb back to NULL when we have freed it in the Rx cleanup
path.  I found one spot where this wasn't occurring so this patch fixes it.

Change-ID: Iaca68934200732cd4a63eb0bd83b539c95f8c4dd
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Orabug: 26132523
(cherry picked from commit 741b8b832a57402380be79d7d11a59eaf57fff3b)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Brian Maly <brian.maly@oracle.com>