wpa_priv: Clear extended_capa pointers

wpa_priv: Clear extended_capa pointers

For now, there is no support for passing extended_capa pointers through
the driver_privsep.c interface from wpa_priv. Avoid leaving bogus
pointers by explicitly clearing these on both wpa_priv and
wpa_supplicant sides.

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