mac80211: fix NULL pointer dereference in ieee80211_compatible_rates

Fix a possible NULL pointer dereference in ieee80211_compatible_rates
introduced in the patch "mac80211: fix association with some APs". If no bss
is available just use all supported rates in the association request.

Signed-off-by: Helmut Schaa <hschaa@suse.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>