iwlagn: check beacon frame size

When the beacon_skb is NULL, we might still
attempt to use it in this code path (if we
ever get here) -- make the code a bit more
defensive and check the return value of
iwl_fill_beacon_frame() against zero.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>