i40e: don't allocate zero size

Shockingly, the compiler didn't flag this uninitialized variable. This
fixes a potential memory corruption condition where ARQ messages are
written to random memory locations.

Change-Id: Iac82f4562d2bf3f42df3f3b2163d9cbed2160135
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>