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 <[e-mail]>
Cc: [e-mail] # 3.10
Signed-off-by: Felix Fietkau <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/rc80211_minstrel.c

···
 	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;
 
···
 			(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;