hostapd: Make sure hapd->drv_priv gets cleared on driver deinit

hostapd: Make sure hapd->drv_priv gets cleared on driver deinit

Couple of code paths in hostapd.c could have left hapd->drv_priv
pointing to memory that was freed in driver_nl80211.c when a secondary
BSS interface is removed. This could result in use of freed memory and
segfault when the next driver operation (likely during interface
deinit/removal). Fix this by clearing hapd->drv_priv when there is
reason to believe that the old value is not valid within the driver
wrapper anymore.

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