i40iw: Add NULL check for puda buffer

i40iw_puda_get_listbuf may return NULL if the list is empty.
Add NULL check prior to accessing the pointer.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>