mac80211_hwsim: avoid NULL access

There's a race condition -- started can be set to true
before channel is set due to the way mac80211 callbacks
currently work (->start should probably pass the channel
we would like to have initially). For now simply add a
check to hwsim to avoid dereferencing the NULL channel
pointer.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>