wireless, wl1251: Fix potential NULL pointer dereference in wl1251_op_bss_info_changed()

In drivers/net/wireless/wl1251/main.c:wl1251_op_bss_info_changed() we make
a call to ieee80211_beacon_get() which may return NULL, but we do not
check the return value before dereferencing the pointer.
Signed-off-by: NJesper Juhl <jj@chaosbits.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/wl1251/main.c

@@ -1039,6 +1039,9 @@ static void wl1251_op_bss_info_changed(struct ieee80211_hw *hw,
 
 	if (changed & BSS_CHANGED_BEACON) {
 		beacon = ieee80211_beacon_get(hw, vif);
+		if (!beacon)
+			goto out_sleep;
+
 		ret = wl1251_cmd_template_set(wl, CMD_BEACON, beacon->data,
 					      beacon->len);