Fix hostapd operation without hw_mode driver data

Fix hostapd operation without hw_mode driver data

Commit 7f0303d5b0bb425f3e7318a7016b55ba9e67f9de ('hostapd: Verify VHT
160/80+80 MHz driver support') added couple of hapd->iface->current_mode
dereferences of which the one in hostapd_set_freq() can be hit with some
configuration files when using driver wrappers that do not have hw_mode
data, i.e., when current_mode is NULL. This could result in segmentation
fault when trying to use driver=wired. Fix this by checking that
current_mode is not NULL before dereferencing it to get vht_capab.

Signed-off-by: Jouni Malinen <j@w1.fi>