i40e: Fix a few potential VF dereferences

In some functions we might be doing potential dereference
without a check. This patch puts the check in place for all these
functions. Also fix the "for loops" so that we increment VF at the
right place so that we always do it even if we are short-circuiting
the loop through continue.

Change-ID: Id4276cfb1e841031bb7b6d6790c414242f364a9f
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>