mac80211/minstrel: fix NULL pointer dereference issue

When priv_sta == NULL, mi->prev_sample is dereferenced too early. Move
the assignment further down, after the rate_control_send_low call.
Reported-by: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: stable@vger.kernel.org # 3.10
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.c

@@ -290,7 +290,7 @@ minstrel_get_rate(void *priv, struct ieee80211_sta *sta,
 	struct minstrel_rate *msr, *mr;
 	unsigned int ndx;
 	bool mrr_capable;
-	bool prev_sample = mi->prev_sample;
+	bool prev_sample;
 	int delta;
 	int sampling_ratio;
 
@@ -314,6 +314,7 @@ minstrel_get_rate(void *priv, struct ieee80211_sta *sta,
 			(mi->sample_count + mi->sample_deferred / 2);
 
 	/* delta < 0: no sampling required */
+	prev_sample = mi->prev_sample;
 	mi->prev_sample = false;
 	if (delta < 0 || (!mrr_capable && prev_sample))
 		return;