ath9k: stop on rates with idx -1 in ath9k rate control's .tx_status

Rate control algorithms are supposed to stop processing when they
encounter a rate with the index -1.  Checking for rate->count not being
zero is not enough.

Allowing a rate with negative index leads to memory corruption in
ath_debug_stat_rc().

One consequence of the bug is discussed at
https://bugzilla.redhat.com/show_bug.cgi?id=768639

Signed-off-by: Pavel Roskin <proski@gnu.org>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -1346,7 +1346,7 @@ static void ath_tx_status(void *priv, struct ieee80211_supported_band *sband,
 	fc = hdr->frame_control;
 	for (i = 0; i < sc->hw->max_rates; i++) {
 		struct ieee80211_tx_rate *rate = &tx_info->status.rates[i];
-		if (!rate->count)
+		if (rate->idx < 0 || !rate->count)
 			break;
 
 		final_ts_idx = i;