mac80211: fix invalid band deref building preq IEs
The function building probe-request IEs does not validate the band is
supported before dereferencing it. This can result in a panic when
all bands are traversed, as done during sched-scan start.
Warn when this happens and return an empty probe request. Also fix
sched-scan to not waste memory on unsupported bands.
Signed-off-by: Arik Nemtsov <arik@wizery.com> Signed-off-by: Johannes Berg <johannes.berg@intel.com>