mac80211: check vif pointer before airtime calculation

In case of monitor mode injection, vif may be NULL, don't crash
on that in ieee80211_calc_expected_tx_airtime().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20200221104544.dddb7a3568fd.I0ede2733a3c76e95daeab07538449ea847e7b78d@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>