ath9k: check for NULL rate when using ieee80211_get_rts_cts_rate()

ieee80211_get_rts_cts_rate() can return NULL, so don't rely
on its members when it does return NULL.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>