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>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -2006,6 +2006,9 @@ static void setup_frame_info(struct ieee80211_hw *hw,
 		fi->keyix = ATH9K_TXKEYIX_INVALID;
 	fi->keytype = keytype;
 	fi->framelen = framelen;
+
+	if (!rate)
+		return;
 	fi->rtscts_rate = rate->hw_value;
 	if (short_preamble)
 		fi->rtscts_rate |= rate->hw_value_short;