iwlwifi: mvm: fix potential NULL-dereference in iwl_mvm_reorder()

We try to access sta before we check for IS_ERR_OR_NULL(), so we may
end up accessing a NULL pointer.  To prevent that, move the conversion
from sta to mvm_sta below the check.

Fixes: b915c10174fb ("iwlwifi: mvm: add reorder buffer per queue")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c

@@ -581,7 +581,7 @@ static bool iwl_mvm_reorder(struct iwl_mvm *mvm,
 			    struct iwl_rx_mpdu_desc *desc)
 {
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
-	struct iwl_mvm_sta *mvm_sta = iwl_mvm_sta_from_mac80211(sta);
+	struct iwl_mvm_sta *mvm_sta;
 	struct iwl_mvm_baid_data *baid_data;
 	struct iwl_mvm_reorder_buffer *buffer;
 	struct sk_buff *tail;
@@ -604,6 +604,8 @@ static bool iwl_mvm_reorder(struct iwl_mvm *mvm,
 	if (WARN_ON(IS_ERR_OR_NULL(sta)))
 		return false;
 
+	mvm_sta = iwl_mvm_sta_from_mac80211(sta);
+
 	/* not a data packet */
 	if (!ieee80211_is_data_qos(hdr->frame_control) ||
 	    is_multicast_ether_addr(hdr->addr1))