i40evf: protect against NULL msix_entries and q_vectors pointers

Update the functions which free msix_entries and q_vectors so that they
are safe against NULL values. This allows calling code to not care
whether these have already been freed when disabling and freeing them.

Change-ID: I31bfd1c0da18023d971b618edc6fb049721f3298
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>