mac80211: minstrel: avoid accessing negative indices in rix_to_ndx()

If rix is not found in mi->r[], i will become -1 after the loop.  This value
is eventually used to access arrays, so we were accessing arrays with a
negative index, which is obviously not what we want to do.  This patch fixes
this potential problem.
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel.c

@@ -66,7 +66,7 @@ rix_to_ndx(struct minstrel_sta_info *mi, int rix)
 	for (i = rix; i >= 0; i--)
 		if (mi->r[i].rix == rix)
 			break;
-	WARN_ON(mi->r[i].rix != rix);
+	WARN_ON(i < 0);
 	return i;
 }
 
@@ -181,6 +181,9 @@ minstrel_tx_status(void *priv, struct ieee80211_supported_band *sband,
 			break;
 
 		ndx = rix_to_ndx(mi, ar[i].idx);
+		if (ndx < 0)
+			continue;
+
 		mi->r[ndx].attempts += ar[i].count;
 
 		if ((i != IEEE80211_TX_MAX_RATES - 1) && (ar[i + 1].idx < 0))