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: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>