mac80211: minstrel_ht: fix out-of-bound in minstrel_ht_set_best_prob_rate

Patch fixes this splat

BUG: KASAN: slab-out-of-bounds in minstrel_ht_update_stats.isra.7+0x6e1/0x9e0
[mac80211] at addr ffff8800cee640f4 Read of size 4 by task swapper/3/0

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Link: http://lkml.kernel.org/r/CALYGNiNyJhSaVnE35qS6UCGaSb2Dx1_i5HcRavuOX14oTz2P+w@mail.gmail.com


Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>