i40e: Fix to check for NULL

This patch fixes an issue in the virt channel code, where a return
from i40e_find_vsi_from_id was not checked for NULL when applicable.
Without this patch, there is a risk for panic and static analysis
tools complain. This patch fixes the problem by adding the check
and adding an additional input check for similar reasons.

Change-ID: I7e9be88eb7a3addb50eadc451c8336d9e06f5394
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>