iwlegacy: 4965-rs: avoid null pointer dereference error

il4965_rs_initialize_lq checks to see if sta is null, however, before that
check il4965_rs_use_green dereferences sta when intializing use_green.
Avoid a potential null pointer dereference error by only calling
il4965_rs_use_green after we are sure sta is not null.

Smatch analysis:

drivers/net/wireless/iwlegacy/4965-rs.c:2160 il4965_rs_initialize_lq() warn:
  variable dereferenced before check 'sta' (see line 2155)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlegacy/4965-rs.c

@@ -2152,14 +2152,15 @@ il4965_rs_initialize_lq(struct il_priv *il, struct ieee80211_conf *conf,
 	int rate_idx;
 	int i;
 	u32 rate;
-	u8 use_green = il4965_rs_use_green(il, sta);
+	u8 use_green;
 	u8 active_tbl = 0;
 	u8 valid_tx_ant;
 	struct il_station_priv *sta_priv;
 
 	if (!sta || !lq_sta)
 		return;
 
+	use_green = il4965_rs_use_green(il, sta);
 	sta_priv = (void *)sta->drv_priv;
 
 	i = lq_sta->last_txrate_idx;