cfg80211: protect fools returning NULL in add_virtual_intf

Callback add_virtual_intf is supposed to return ERR_PTR and trying to
return NULL results in some "Unable to handle kernel paging request",
etc. As it may be complicated to debug & trace, let's catch it (WARN).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>