mac80211: minstrel: fix tx status processing corner case

Some drivers fill the status rate list without setting the rate index after
the final rate to -1. minstrel_ht already deals with this, but minstrel
doesn't, which causes it to get stuck at the lowest rate on these drivers.

Fix this by checking the count as well.

Cc: stable@vger.kernel.org
Fixes: cccf129 ("mac80211: add the 'minstrel' rate control algorithm")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20201111183359.43528-3-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.c

@@ -274,7 +274,7 @@ minstrel_tx_status(void *priv, struct ieee80211_supported_band *sband,
 	success = !!(info->flags & IEEE80211_TX_STAT_ACK);
 
 	for (i = 0; i < IEEE80211_TX_MAX_RATES; i++) {
-		if (ar[i].idx < 0)
+		if (ar[i].idx < 0 || !ar[i].count)
 			break;
 
 		ndx = rix_to_ndx(mi, ar[i].idx);