Fix a segfault in wpa_supplicant_deinit
If wpa_supplicant is started with -u but the DBus service is already
registered wpa_supplicant will bail out. However, it will segfault
in wpa_supplicant_deinit because global->drv_priv wasn't allocated
yet.
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>