wil6210: fix _desc access in __wil_tx_vring_tso

_desc is defined in __wil_tx_vring_tso() and may not be set in
case len is 0, verify _desc is set.
Signed-off-by: NAlexei Avshalom Lazar <ailizaro@codeaurora.org>
Signed-off-by: NMaya Erez <merez@codeaurora.org>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/wil6210/txrx.c

@@ -1761,6 +1761,9 @@ static int __wil_tx_vring_tso(struct wil6210_priv *wil, struct wil6210_vif *vif,
 		}
 	}
 
+	if (!_desc)
+		goto mem_error;
+
 	/* first descriptor may also be the last.
 	 * in this case d pointer is invalid
 	 */