mac80211_hwsim: check txrate for NULL

If the rate control algorithm messed up then the txrate pointer
here could be NULL - WARN and drop the packet from monitoring.

Signed-off-by: Amit Khatri <amit.khatri@samsung.com>
Signed-off-by: Rahul Jain <rahul.jain@samsung.com>
[rewrite commit message, add warning]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>