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: NWojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/main.c

@@ -513,7 +513,7 @@ irqreturn_t ath_isr(int irq, void *dev)
 	 * 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 */