rt2x00: Fix NULL pointer error in adhoc/master mode

As soon as an interface is enabled, and that interface is in adhoc or master mode,
the device will start raising beacondone interrupts. But before the first interrupt is
raised, mac80211 will probably not have send any beacons to the device yet, which
results in a NULL pointer error when the skb is being freed.

Note that the "raise beacondone interrupts without a beacon" is also a bug,
and will be addressed later. The more important bug however is preventing
the NULL pointer failt itself, since there might be other conditions that could trigger
it as well.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>