i40evf: check for msix_entries null dereference

It is possible for msix_entries to be freed by a previous suspend/remove
before a VF is closed.  This patch fixes the issue by checking for NULL
before dereferencing msix_entries and returning early in the case where
it is NULL within the i40evf_close code path.  Without this patch it is
possible to trigger a kernel panic through NULL dereference.

Change-ID: I92a2746e82533a889e25f91578eac9abd0388ae2
Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>