greybus: gpio: fix null-deref on unexpected irq requests

Fix null-pointer dereference on failure to look up irq due to missing
error handling.
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/gpio.c

@@ -425,7 +425,15 @@ static void gb_gpio_request_recv(u8 type, struct gb_operation *op)
 		return;
 	}
 	irq = gpio_to_irq(ggc->chip.base + event->which);
+	if (irq < 0) {
+		dev_err(ggc->chip.dev, "failed to map irq\n");
+		return;
+	}
 	desc = irq_to_desc(irq);
+	if (!desc) {
+		dev_err(ggc->chip.dev, "failed to look up irq\n");
+		return;
+	}
 
 	/* Dispatch interrupt */
 	local_irq_disable();