ath9k: Fix NULL pointer dereference on early irq

The ah struct might not have been initialized when
interrupt comes so check for it.

Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/main.c b/drivers/net/wireless/ath/ath9k/main.c
index e6ac8d2..4b148bb 100644
--- a/drivers/net/wireless/ath/ath9k/main.c
+++ b/drivers/net/wireless/ath/ath9k/main.c
@@ -513,7 +513,7 @@
 	 * touch anything. Note this can happen early
 	 * on if the IRQ is shared.
 	 */
-	if (test_bit(ATH_OP_INVALID, &common->op_flags))
+	if (!ah || test_bit(ATH_OP_INVALID, &common->op_flags))
 		return IRQ_NONE;
 
 	/* shared irq, not for us */